Wallop Charlie, an aggressive chap (5)
I believe the answer is:
clout
'wallop' is the definition.
(thesaurus)
'charlie an aggressive chap' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet: alpha, bravo, charlie etc.).
'an aggressive chap' becomes 'lout' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'c'+'lout'='CLOUT'
Can you help me to learn more?
(Other definitions for clout that I've seen before include "A blow; powerful influence" , "Hit - influence" , "political muscle" , "Cuff" , "Ne'er cast a ... till May be out, it's said" .)