Do not want to say that they are the second choice. You can certainly find ways to tell them that you are not ready to respond.
First, evaluate the first offer by itself. Be disciplined in the evaluation ... examine that offer on it's merits. Absent the alternative possibility, would you accept the offer? The company that stepped up an offered deserves some consideration ahead of the company that can't get you an answer or is stringing you along.
--> If not OK, decline.
If it is OK but you are hoping for better, things are more complex.
Is the offer FINAL?
If the offer has contingencies (i.e background check, references, etc.), a perfectly suitable response is something like "The offer is acceptable in principle, and when the contingencies are removed and the offer is finalized, I will be able to make a commitment."
Ask for more time
As Alexa noted, asking for more time is reasonable. I would not necessarily do so if you have already created significant delay for other reasons. You want to avoid the perception that you are stalling.
Don't ask for excessive time .. ask for a week at most. And once you do so, don't do it again.
See if you can find some ways to delay ... there are lots. A good one is to ask for the paperwork that you will be required to sign, and then tell them you need some time to review with counsel or an adviser. Ditto for the employee handbook and other binding documents. Ditto for the specific benefit information.
Be careful .. you only get one or two of these before it looks bad.
Note: do not NEGOTIATE in an attempt to delay. That can backfire.
"A bird in the hand is worth two in the bush."
If the company with the outstanding offer demands a response and the preferred company has not offered, accept. You can always decline later or resign if a better offer comes in.
I used to think that this was bad faith, but my mind has changed due to corporate behavior in the last 15 years. The offering company feels no special obligation to you. Most companies will not hesitate to rescind the offer, cancel the position, or lay you off if they determine necessary. You owe them no special loyalty.
--> Very likely, the company stretching it out has you as their second choice and are waiting on a first choice to answer.
General notes about timing
Just as you have other opportunities in the hopper, the companies have other candidates. You do owe all them good faith. As soon as you have ruled out a company, tell them.
Include how the companies have treated you in your evaluation. Essentially, "points" for having a good process, making a decision, and acting. If a company can't make up their mind and act, or is dismissive, or doesn't return emails or phone calls, that generally means something.
Fish or cut bait.
This can't go on forever. At some point you are going to have to decide. Be aware of this. My suggestion is to bias toward the offer in hand rather than to the possible offer from another company.