Back an attempt (3)
I believe the answer is:
ago
'back' is the definition.
('years back' means 'years ago')
'an attempt' is the wordplay.
'an' becomes 'a'.
'attempt' becomes 'go' (go is a kind of attempt).
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "Previously" , "In past times" , "(Years) before the present time" , "Earlier, gone by" , "Back (in time)" .)