Fuller cow, I fancy, having eaten a vegetable (11)
I believe the answer is:
cauliflower
'vegetable' is the definition.
(cauliflower is a kind of vegetable)
'fuller cow i fancy having eaten a' is the wordplay.
'fancy' indicates anagramming the letters.
'having eaten' is an insertion indicator.
'fuller'+'cow'+'i'='fullercowi'
'fullercowi' with letters rearranged gives 'culiflower'.
'culiflower' enclosing 'a' is 'CAULIFLOWER'.
(Other definitions for cauliflower that I've seen before include "something to eat" , "curds" , "Battle-scarred lug" , "Auricular deformation seen in boxers and rugby forwards?" , "Large vegetable of cabbage family" .)