Scrap spring not opening (5)
I believe the answer is:
ounce
'scrap' is the definition.
(I've seen this before)
'spring not opening' is the wordplay.
'spring' becomes 'pounce' (pounce is a kind of spring).
'not opening' means to remove the first letter (the opening letter is removed).
'pounce' with its first letter taken off is 'OUNCE'.
(Other definitions for ounce that I've seen before include "not weighing very much" , "Snow-leopard" , "Small quantity - snow leopard" , "Unit of weight - snow leopard" , "Wild cat - small weight" .)