Recruited eldest in cooking (8)
I believe the answer is:
enlisted
'recruited' is the definition.
(enlisting is a kind of recruiting)
'eldest in cooking' is the wordplay.
'cooking' indicates an anagram.
'eldest'+'in'='eldestin'
'eldestin' anagrammed gives 'ENLISTED'.
(Other definitions for enlisted that I've seen before include "Joined up (milit.)" , "Lower ranks in US army" , "Joined forces" , "Recruited" , "Persuaded to serve" .)