I heard word of assent (3)
I believe the answer is:
aye
'word of assent' is the definition.
(I've seen this in another clue)
'i heard' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'i' is a homophone of 'AYE'.
(Other definitions for aye that I've seen before include "Indeed" , "Yes; ever" , "An affirmative vote" , "Entirely one" , "Affirmative answer" .)