Cereal crop initially wanting warmth (5)
I believe the answer is:
wheat
'cereal crop' is the definition.
(I know that wheat is a type of cereal crop)
'initially wanting warmth' is the wordplay.
'initially' says to take the initial letters.
'warmth' becomes 'heat' (synonyms).
The initial letter of 'wanting' is 'w'.
'w'+'heat'='WHEAT'
(Other definitions for wheat that I've seen before include "Cereal crop" , "Annual grass with light brown grains" , "Tall grass producing light brown grain" , "Grain crop" , "A cereal, to be separated from chaff" .)