Vegetable many, but not all, leave (9)
I believe the answer is:
mangetout
'vegetable' is the definition.
(I've seen this before)
'many but not all leave' is the wordplay.
'but not all' means to remove the last letter.
'leave' becomes 'getout' (synonyms).
'many' with its last letter taken off is 'man'.
'man'+'getout'='MANGETOUT'
(Other definitions for mangetout that I've seen before include "From this pod" , "something on the plate" , "Sugar peas" , "Type of pea" , "Vegetable" .)