What kind of song are you looking for? Hopefully, they will answer with something more than just “a hit”, but unfortunately that’s the most common answer I get from the artists. Their response to this question can tell you a lot about the song you should write if they are willing to share. Best case scenario is when they say something like “I need an up-tempo song like blah blah that talks about blah blah”. If they give you a precise answer, then you have a much better chance of writing specifically what they need.