How mate's changed a bit (8)
I believe the answer is:
somewhat
'a bit' is the definition.
(I've seen this before)
'how mate's changed' is the wordplay.
'changed' indicates an anagram.
'how'+'mates'='howmates'
'howmates' is an anagram of 'SOMEWHAT'.
(Other definitions for somewhat that I've seen before include "To some degree, sort of" , "Moderately" , "To an extent, a little" , "To a certain extent" , "To a moderate extent" .)