Former purchase, we hear, good, unique (6)
I believe the answer is:
bygone
'former' is the definition.
(thesaurus)
'purchase we hear good unique' is the wordplay.
'purchase' becomes 'buy' ('buy' can be a synonym of 'purchase').
'we hear' shows a homophone (sound like).
'good' becomes 'g' (abbreviation).
'unique' becomes 'one' ('one' can be a synonym of 'unique').
'buy' is a homophone of 'by'.
'by'+'g'+'one'='BYGONE'
(Other definitions for bygone that I've seen before include "Previous" , "Past (e.g. age)" , "Times past, poetically" , "forgotten" , "Erstwhile" .)