A try in the past (3)
I believe the answer is:
ago
'in the past' is the definition.
(three years ago means three years in the past)
'a try' is the wordplay.
'try' becomes 'go' (both can mean an attempt).
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "Back (in time)" , "Earlier, gone by" , "Time past" , "Previously" , "(Years) before the present time" .)