A board game 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 board game' is the wordplay.
'board game' becomes 'go' (go is a kind of board game).
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "Earlier, gone by" , "In past times" , "Gone, past, since" , "Back (in time)" , "Time past" .)