As it's for a solicitor position, I would imagine they mean "challenging" as in violent? Or maybe "not all there"?
You could start by saying that all individuals are challenging per se, as every case is bound to be different from the next one. after all, there isn't a job more varied and challenging that one where you deal with people? You could maybe point out that this is exactly why you chose this profession, as you love challenges and are very eager to learn and go through new experiences in your professional life?
Then go on to say that, when it comes to someone particularly "challenging", you would try to listen as much as possible, to understand what they mean to say, because listening is the most important part of a professional client relatonship....then try to lay down the fact logically, leaving out the inherent emotional side?
No idea really...lol Hope that helped a wee bit?