Proposition that he or employer will accept (7)
I believe the answer is:
theorem
'proposition' is the definition.
(I know that theorem is a type of proposition)
'that he or employer will accept' is the wordplay.
'will accept' indicates the answer is hidden within the clue.
'THEOREM' can be found hidden inside 'that he or employer'.
(Other definitions for theorem that I've seen before include "The more it changed to a proposition to be proved" , "speculation" , "Deduced formula" , "Hero met (anag) - principle that's proven" , "Proved mathematical hypothesis" .)