One cabbage (4)
I believe the answer is:
head
'one cabbage' is the definition.
(head cabbage is a kind of cabbage)
This is the entire clue.
(Other definitions for head that I've seen before include "Lead - school principal" , "Edible part of cabbage" , "''and still the wonder grew / That one small ... could carry all he knew'' (Goldsmith)" , "Leader; stream source" , "John on board" .)