Cereal we hate, mostly when cooked (5)
I believe the answer is:
wheat
'cereal' is the definition.
(wheat is a kind of cereal)
'we hate mostly when cooked' is the wordplay.
'mostly' means to remove the last letter.
'when cooked' is an anagram indicator.
'we'+'hate'='wehate'
'wehate' with its last letter taken off is 'wehat'.
'wehat' is an anagram of 'WHEAT'.
(Other definitions for wheat that I've seen before include "Source of flour" , "Annual grass with light brown grains" , "Bread ingredient" , "A cereal, to be separated from chaff" , "Type of cereal" .)