Hit boy below the belt (4)
I believe the answer is:
blow
'hit' is the definition.
(I know that hit can be written as blow)
'boy below the belt' is the wordplay.
'boy' becomes 'b' (abbreviation used in schools).
'below the belt' becomes 'low' (below the belt can mean morally low).
'b'+'low'='BLOW'
(Other definitions for blow that I've seen before include "Use pipe for" , "Waste" , "Cease to be a problem" , "Shock; sound (a horn)" , "Bang, wallop" .)