Heavy blow caught oaf (5)
I believe the answer is:
clout
'heavy blow' is the definition.
(I've seen this before)
'caught oaf' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'oaf' becomes 'lout' (synonyms).
'c'+'lout'='CLOUT'
(Other definitions for clout that I've seen before include "Whack" , "authority" , "Swipe" , "Smack - influence" , "Box" .)