they have requested I go through my client list with them to see
crossover potential. How do I protect myself but at the same time not
appear to be difficult? I am a team player, but without a contract
being signed with them am unsure that handing them a client list is in
my best interest.
There is no way I would give a client list from my current employer to a company with whom I am not employed.
First, your current contract may prohibit such a move.
And second, even if it isn't prohibited by your contract you simply cannot give away a valuable asset to someone who may or may not become your employer.
Ignoring the ethicality (or lack of ethicality) in this situation, you shouldn't give any clients' information to a non-employer.
You could offer to go through your list on your own, match it up with their list, and give them a number of clients that might potentially become their client. No names, just the count of clients. And on your own, not "with them". Perhaps they would like to give away their client list to someone who isn't yet an employee, perhaps not. (I suspect not.)
If your current contract allows, you could suggest that the client names would of course follow, once you are hired.
Depending on your current contract, and the norms within your industry and your locale, you need to decide if this is ethical, and if not, if this is the kind of company you would want to work for. It smells foul to me, but I understand in some domains this is rather common practice.