Opener for Arsenal -- on top of game since (3)
I believe the answer is:
ago
'since' is the definition.
(I've seen this before)
'opener for arsenal on top of game' is the wordplay.
'opener for' suggests taking the first letters.
'on top of' says to put letters next to each other.
'game' becomes 'go' (go is a board game).
The initial letter of 'arsenal' is 'a'.
'a'+'go'='AGO'
(Other definitions for ago that I've seen before include "Back (in time)" , "Time past" , "In past times" , "Earlier, gone by" , "Gone, past, since" .)