Chum changed considerably (4)
I believe the answer is:
much
'considerably' is the definition.
(I've seen this in another clue)
'chum changed' is the wordplay.
'changed' indicates an anagram.
'chum' with letters rearranged gives 'MUCH'.
(Other definitions for much that I've seen before include "Plenty" , "Large quantity" , "Chum (anag.)" , "A large amount" , "Great quantity" .)