As mentioned in my comment, I think most people understand the concept of a portfolio, especially if you specifically state that you are a front end developer. That said, there are some steps you are able to take to reduce the chances of misunderstandings or legal problems.
Be honest, and be able to prove it
Don't say anything that is untrue, and ensure that everything you say is clearly, demonstrably true. By this I mean that you should be able to back up the fact that what you say is true, if anyone ever asked. If you're able to prove that you were in fact the developer, you shouldn't have any trouble.
Be clear and concise... but mostly be clear.
If there's any ambiguity, clear it up. Don't let what you don't say, hint at something more than is true. That doesn't mean you have to explain absolutely everything, but it's more important to ensure that you are very clear about what you were/weren't responsible for.
I find a good way to do this is to have a set of icons under each "portfolio" item, with a legend on the side. A little icon for each of the following, greyed out if they aren't true or coloured if they are true, for example (although you may want more concise descriptions, I'm including the full sentences for your benefit)
- I designed this website graphically
- I was the front end developer for this website (eg I did the code for the design/interface)
- I developed the entire front and back end for this website (eg I made the whole thing work)
- I created this website as a freelancer working for the owner
- I created this website as an employee of the owner
- I own this website
- I created this website voluntarily
- I was paid to create this website
You may want to add to/remove from this list depending on your specific situation, but this idea is to have small, unobtrusive items under each portfolio entry such that any reader can look into the details and know exactly how much credit/responsibility you are taking for that design.
Some people may just want to see some things you've worked on to check they look professional, others will want to know exactly what was or wasn't you. It also covers you from a "You were trying to mislead people" or "You were trying to claim ownership" perspective.
When clicking through from the main page, you may want to highlight the descriptions of the above icons on the "details" page - ie a list of just the "highlighted" icons, with their descriptions, to show exactly what you did or didn't do.
And finally on the details page, a quick explanation paragraph goes a long way to making it clear exactly what you are/aren't saying, while giving you a chance to sell yourself
If in doubt, ask or don't include it
Unless you're very new to your industry, you likely have a number of examples of work. That means you can afford to not include one or two where you're worried there may be legal or moral issues, or simply where you aren't totally sure what the owner would be happy with or cannot prove your involvement.
Initially you can try to ask the owner what they're happy with, and show them what you will include on the website (so that they can raise issues with it if they wish), and perhaps ask for a quick note of documentary evidence to show that you were involved to that extent, and that they're happy for it to be included. If you can't get this, however, just leave it out. You can't get in trouble for leaving out one or two examples, and you should have enough from the rest of your portfolio to flesh it out anyway.