Dressed as a bounder about fifty (4)
I believe the answer is:
clad
'dressed' is the definition.
(to be clad in something is to be dressed in it)
'a bounder about fifty' is the wordplay.
'a bounder' becomes 'cad' (synonyms).
'about' means one lot of letters goes inside another.
'fifty' becomes 'L' (Roman numeral).
'cad' going around 'l' is 'CLAD'.
'as' acts as a link.
(Other definitions for clad that I've seen before include "Provided with covering" , "Invested" , "Cover with insulation" , "Clothed, covered" , "Decked" .)