A pass back (3)
I believe the answer is:
ago
'back' is the definition.
('years back' means 'years ago')
'a pass' is the wordplay.
'pass' becomes 'go' (synonyms).
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "In past times" , "Back (in time)" , "Gone, past, since" , "Time past" , "Previously" .)