A journey back (3)
I believe the answer is:
ago
'back' is the definition.
('ago' can be a synonym of 'back')
'a journey' is the wordplay.
'journey' becomes 'go' (I've seen this in other clues).
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "Back (in time)" , "In past times" , "Time past" , "Gone, past, since" , "Earlier, gone by" .)