Discover sir with purse for a change (8)
I believe the answer is:
surprise
'discover' is the definition.
(thesaurus)
'sir with purse for a change' is the wordplay.
'with' is a charade indicator (letters next to each other).
'for a change' is an anagram indicator.
'sir' after 'purse' is 'pursesir'.
'pursesir' anagrammed gives 'SURPRISE'.
(Other definitions for surprise that I've seen before include "Unexpected thing" , "Shock" , "Sudden unexpected event" , "Bombshell" , "One of spelt out words" .)