Cereal grass slightly bitter, we hear? (3)
I believe the answer is:
rye
'cereal grass' is the definition.
(I know that rye is a type of cereal grass)
'slightly bitter we hear?' is the wordplay.
'slightly bitter' becomes 'wry' (wry can mean bitter or sardonic).
'we hear?' indicates a 'sounds like' (homophone) clue.
'wry' is a homophone of 'RYE'.
(Other definitions for rye that I've seen before include "E. Sussex town; cereal" , "Grass related to wheat" , "Whisky; Sussex town" , "Drink" , "Type of whisky" .)