To retaliate, or get even, a counterfeit note is put in (6)
I believe the answer is:
avenge
'to retaliate' is the definition.
(synonyms)
'get even a counterfeit note is put in' is the wordplay.
'get' says to put letters next to each other.
'counterfeit' indicates an anagram.
'note' becomes 'g' (musical note).
'is put in' means one lot of letters goes inside another.
'even' next to 'a' is 'evena'.
'evena' anagrammed gives 'avene'.
'avene' enclosing 'g' is 'AVENGE'.
'or' is the link.
(Other definitions for avenge that I've seen before include "Get one's own back for" , "Gain retribution" , "Redress, vindicate" , "Inflict harm in return (for wrong)" , "Carry out retribution" .)