Scratch small part of apple (5)
I believe the answer is:
score
'scratch' is the definition.
(score is a kind of scratch)
'small part of apple' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'part of apple' becomes 'core'.
's'+'core'='SCORE'
(Other definitions for score that I've seen before include "Group of 20" , "Nick; twenty" , "Musical composition in written form" , "purchase of illegal drugs" , "record" .)