One pushing for union membership expected to bend the knee (8)
I believe the answer is:
proposer
'one pushing for union membership expected to bend the knee' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one pushing for' is the wordplay.
I cannot quite understand how this works, but
'for' could be 'pro' (as in being 'pro' something) and 'pro' is present in the answer.
The remaining letters 'poser' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for proposer that I've seen before include "One introducing motion" , "Advocate" , "One hopes to be engaged" , "he asks for a hand" , "One putting forward motion" .)