Redeem a forged note (5)
I believe the answer is:
atone
'redeem' is the definition.
(thesaurus)
'a forged note' is the wordplay.
'forged' is an anagram indicator.
'note' is an anagram of 'tone'.
'a'+'tone'='ATONE'
(Other definitions for atone that I've seen before include "In agreement - sounds singular" , "Pay the price" , "Make up for wrong done" , "Make good" , "remedy" .)