Cook possibly eats a lot (5)
I believe the answer is:
steam
'cook' is the definition.
(steaming is a kind of cooking)
'possibly eats a lot' is the wordplay.
'possibly' indicates an anagram.
'a lot' becomes 'm' ().
'eats' anagrammed gives 'stea'.
'stea'+'m'='STEAM'
(Other definitions for steam that I've seen before include "Vapour from water at boiling point" , "A lot of hot air" , "Speed" , "Hot water vapour" , "Engine driver" .)