They express assent (4)
I believe the answer is:
ayes
'they express assent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ayes that I've seen before include "Nautical positive responses" , "They express assent" , "Those voting for the motion" , "Affirmative votes as in committee" , "Sounds of agreement" .)