A friend much altered (4)
I believe the answer is:
chum
'a friend' is the definition.
(I know that chum is a type of friend)
'much altered' is the wordplay.
'altered' indicates anagramming the letters.
'much' with letters rearranged gives 'CHUM'.
(Other definitions for chum that I've seen before include "Good friend" , "Close friend (colloq.)" , "Salmon" , "Pal, buddy" , "Pal, mate" .)