A turn previously (3)
I believe the answer is:
ago
'previously' is the definition.
(I've seen this before)
'a turn' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "Gone, past, since" , "(Years) before the present time" , "In past times" , "Time past" , "Previously" .)