Crop needing warmth after onset of winter (5)
I believe the answer is:
wheat
'crop' is the definition.
(I know that wheat is a type of cereal)
'warmth after onset of winter' is the wordplay.
'warmth' becomes 'heat' (synonyms).
'after' means one lot of letters go next to another.
'onset of' suggests taking the first letters.
The first letter of 'winter' is 'w'.
'heat' put after 'w' is 'WHEAT'.
'needing' is the link.
(Other definitions for wheat that I've seen before include "cereal's required" , "Source of flour" , "staple food" , "Bread ingredient" , "Tall grass producing light brown grain" .)