Cuff that is small and round (4)
I believe the answer is:
slap
'cuff' is the definition.
(I know that cuff is a more specific form of the action slap)
'small and round' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' is a charade indicator (letters next to each other).
'round' becomes 'lap' (as in a round or lap of a park).
's'+'lap'='SLAP'
'that is' acts as a link.
(Other definitions for slap that I've seen before include "Strike action" , "Hit with an open hand" , "Sharp smack" , "Strike (in the face)" , "Cuff" .)