Observe affirmative response, we hear (3)
I believe the answer is:
eye
'observe' is the definition.
(thesaurus)
'affirmative response we hear' is the wordplay.
'affirmative response' becomes 'aye'.
'we hear' indicates a 'sounds like' (homophone) clue.
'aye' sounds like 'EYE'.
(Other definitions for eye that I've seen before include "Needle or lace hole" , "where the pupil is seated" , "Four [NEEDLEFUL] may pass through this" , "Aperture" , "potato bud" .)