Vegetables - stores surplus supply in empty boxes (8,7)
I believe the answer is:
brussels sprouts
'vegetables' is the definition.
(brussels sprout is a kind of vegetable)
'stores surplus supply in empty boxes' is the wordplay.
'supply' indicates anagramming the letters (supply of letters to make).
'in' means one lot of letters goes inside another.
'empty' says to hollow out the word (remove centre letters).
'boxes' with its centre removed is 'bs'.
'stores'+'surplus'='storessurplus'
'storessurplus' with letters rearranged gives 'russelssprout'.
'russelssprout' placed into 'bs' is 'BRUSSELS SPROUTS'.
(Other definitions for brussels sprouts that I've seen before include "Vegetables" , "part of the traditional 27 [DINNER] - with 8a" , "greens" .)