Perspire when cooking a stew (5)
I believe the answer is:
sweat
'perspire' is the definition.
(synonyms)
'cooking a stew' is the wordplay.
'cooking' indicates anagramming the letters (letters cooked up into a new form).
'a'+'stew'='astew'
'astew' anagrammed gives 'SWEAT'.
'when' acts as a link.
(Other definitions for sweat that I've seen before include "Male perspiration?" , "Soldier" , "Salty fluid secreted by glands" , "Toil" , "Waste (anag.)" .)