"Do not be blocked".
This is so fundamental that I considered a "thou shalt not...."
Don't wait for specifications.
Don't wait for clarifications.
Go with what you have. Influence the decision with what you know and what you can do.
Make functional samples, sketches, proto types. Duct tape is allowed.
Seize ownership. Become integral.
Learn the problem by spending time in the problem domain.
It is allowed to throw away software. Even newly connected neurons is a benefit.
Something tangible and concrete produces tangible and concrete feedback.