A much muddled mate (4)
I believe the answer is:
'mate' is the definition.
(I've seen this before)
'a much muddled' is the wordplay.
'muddled' is an anagram indicator.
'much' anagrammed gives 'CHUM'.
(Other definitions for chum that I've seen before include "Close friend (colloq.)" , "Salmon" , "Informally, a close friend" , "Pal, buddy" , "Pal, mate" .)