Mix up clothing, porter possibly going topless (6)
I believe the answer is:
garble
'mix up' is the definition.
(thesaurus)
'clothing porter possibly going topless' is the wordplay.
'clothing' becomes 'garb' (garb is a kind of clothing**).
'porter possibly' becomes 'ale' (porter is a kind of ale).
'going topless' means to remove the first letter.
'ale' with its first letter taken away is 'le'.
'garb'+'le'='GARBLE'
(Other definitions for garble that I've seen before include "Indistinct speech" , "Distort (message)" , "jumble" , "Confuse (a message)" , "Distort text or its meaning" .)