Thin; left over (5)
I believe the answer is:
spare
I believe this is a double definition.
'thin' is the first definition.
(similar in meaning)
'left over' is the second definition.
(I've seen this in another clue)
(Other definitions for spare that I've seen before include "Leftover" , "Not required" , "Additional; thin" , "Let off; extra" , "In reserve; pardon" .)