Wallop Charlie, the oaf! (5)
I believe the answer is:
clout
'wallop' is the definition.
(thesaurus)
'charlie the oaf' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'the oaf' becomes 'lout' ('lout' can be a synonym of 'oaf').
'c'+'lout'='CLOUT'
(Other definitions for clout that I've seen before include "piece of clothing" , "authority" , "Bash" , "Blow to the head" , "hit hard" .)