Heat the water at the end of winter! (4)
I believe the answer is:
sear
'heat' is the definition.
(I know that sear is a more specific form of the action heat)
'water at the end of winter' is the wordplay.
'water' becomes 'sea' (the sea is water).
'at the end of' indicates one should take the final letters.
The final letter of 'winter' is 'r'.
'sea'+'r'='SEAR'
'the' acts as a link.
(Other definitions for sear that I've seen before include "Burn, scorch intensely" , "Scorch, brand" , "Make very hot and dry" , "Scorch or burn the surface of" , "Cauterise" .)