To Ann, a changed man! (5)
I believe the answer is:
anton
'man' is the definition.
(I've seen this before)
'to ann a changed' is the wordplay.
'a changed' indicates an anagram.
'to'+'ann'='toann'
'toann' with letters rearranged gives 'ANTON'.
(Other definitions for anton that I've seen before include "..... Chekov" , "-- Chekhov, Russian dramatist" , "Chekhov, say" , "-- Bruckner, composer" , "Boy" .)