Label on small heart (7)
I believe the answer is:
sticker
'label' is the definition.
(I know that sticker is a type of label)
'on small heart' is the wordplay.
'on' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'heart' becomes 'ticker' (informal term for the heart).
's' put next to 'ticker' is 'STICKER'.
(Other definitions for sticker that I've seen before include "he soldiers on!" , "Gummed label" , "Adhesive label" , "Determined person" , "Person with staying power" .)