Cook in water (4)
I believe the answer is:
boil
'cook in water' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boil that I've seen before include "Skin swelling; cause to evaporate" , "Sore with a hard core filled with pus" , "Painful sore" , "Cook in hot water" , "Cook, swelling" .)