Possible replacement's in box — leave lid off (5,4)
I believe the answer is:
spare part
'possible replacement's' is the definition.
'box leave lid off' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'leave' becomes 'depart' (departing is a kind of leaving).
'lid off' means to remove the first letter (in a down clue, lid can mean the top letter).
'depart' with its first letter taken off is 'epart'.
'spar'+'epart'='SPARE-PART'
'in' acts as a link.
(Other definitions for spare part that I've seen before include "Extra component for machine" , "Replacement component" , "something put in machine?" , "Type of surgery" .)