Extra, in reserve (5)
I believe the answer is:
spare
This clue is a double definition.
'extra' is the first definition.
(both can mean left over)
'in reserve' is the second definition.
(I've seen this in another clue)
(Other definitions for spare that I've seen before include "In reserve; pardon" , "Let off; extra" , "show mercy" , "Surplus to requirements" , "Don't use" .)