Tire in a trunk (5)
I believe the answer is:
spare
'tire in a trunk' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spare that I've seen before include "Thin; elegantly simple" , "In reserve; pardon" , "Free" , "Refrain from using" , "Pardon; unneeded" .)