Strike small child (5)
I believe the answer is:
smite
'strike' is the definition.
(I've seen this before)
'small child' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'child' becomes 'mite' (mite can informally mean a small child).
's'+'mite'='SMITE'
(Other definitions for smite that I've seen before include "Thwack" , "Beat" , "Whack" , "Hit hard in olden times" , "Strike in archaic way at times" .)