Cereal in west requiring warmth (5)
I believe the answer is:
wheat
'cereal' is the definition.
(I know that wheat is a type of cereal)
'west requiring warmth' is the wordplay.
'west' becomes 'W' (abbreviation).
'requiring' is a charade indicator (letters next to each other).
'warmth' becomes 'heat' (synonyms).
'w'+'heat'='WHEAT'
'in' is the link.
(Other definitions for wheat that I've seen before include "Cereal crop" , "Annual grass with light brown grains" , "Grass whose grain is used to make flour" , "Type of cereal" , "food plant" .)