Friend is much altered (4)
I believe the answer is:
chum
'friend' is the definition.
(chum is a kind of friend)
'much altered' is the wordplay.
'altered' is an anagram indicator.
'much' with letters rearranged gives 'CHUM'.
'is' acts as a link.
(Other definitions for chum that I've seen before include "Salmon" , "Close friend (colloq.)" , "Informally, a close friend" , "Pal, mate" , "Pal, buddy" .)