Leave old food unfinished (5)
I believe the answer is:
exeat
'leave' is the definition.
(I've seen this before)
'old food unfinished' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'food' becomes 'eats' (eats can mean food).
'unfinished' means to remove the last letter.
'eats' with its last letter taken off is 'eat'.
'ex'+'eat'='EXEAT'
(Other definitions for exeat that I've seen before include "Permission to go" , "Permit for absence" , "Pass-out" , "Permission for absence from school" , "Leave of absence" .)