Wallop Charlie, the oaf! (5)
I believe the answer is:
'wallop' is the definition.
'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').
(Other definitions for clout that I've seen before include "Old-fashioned garment", "hit hard", "Cast not one before May is out", "political muscle", "Ne'er cast a ... till May be out, it's said".)