Hit small child (5)
I believe the answer is:
smite
'hit' is the definition.
(I know that smite is a more specific form of the action hit)
'small child' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'child' becomes 'mite' (I've seen this before).
's'+'mite'='SMITE'
(Other definitions for smite that I've seen before include "Hit hard in olden times" , "Strike with a firm blow" , "Beat" , "Strike heavily" , "Thwack" .)