I always ask clients to clarify their instructions if something is unclear. The client usually appreciates the attention to detail and is happy to answer any questions.
If you don't ask, you risk writing something completely incorrect and then you'll just piss off your client and waste your time trying to fix it.
The approach that works best for me is to ask very specific questions instead of just asking the client to explain further. If their instructions were poor to begin with, its likely they won't know what you need unless you tell them.
So ask, ask, ask!
~ gina