Headwear worn by European cook (4)
I believe the answer is:
heat
'cook' is the definition.
(I've seen this before)
'headwear worn by european' is the wordplay.
'headwear' becomes 'hat' (hat is a type of headwear).
'worn by' is an insertion indicator (outside letters are worn by those on the inside).
'european' becomes 'e' (abbreviation e.g. EU).
'hat' placed around 'e' is 'HEAT'.
(Other definitions for heat that I've seen before include "brings out a sweat?" , "Cook" , "Apply warmth" , "Vehemence" , "Preliminary race to qualify finalists" .)