Diane's eager to reform? Beg to differ (8)
I believe the answer is:
disagree
'beg to differ' is the definition.
(I've seen this before)
'diane's eager to reform?' is the wordplay.
'diane's' becomes 'di's' (short for Diane).
'to reform?' indicates an anagram (letters in a new form).
'eager' is an anagram of 'agree'.
'dis'+'agree'='DISAGREE'
(Other definitions for disagree that I've seen before include "source of conflict" , "Differ in opinion" , "Beg to differ" , "Be at odds" , "dispute" .)