Will return to a vegetable (7)
I believe the answer is:
shallot
'a vegetable' is the definition.
(I know that shallot is a type of onion)
'will return to' is the wordplay.
'will' becomes 'shall' (synonyms).
'return' says the letters should be written backwards.
'to' back-to-front is 'ot'.
'shall'+'ot'='SHALLOT'
(Other definitions for shallot that I've seen before include "Host all with kind of onion" , "Scallion" , "bit of salad" , "Type of onion" , "Type of 12" .)