Vault over line, a fair game (7)
I believe the answer is:
Here is my best explanation:
'fair game' is the definition.
(I've seen this before)
'vault over line a' is the subsidiary indication.
'vault' becomes 'tomb' (I've seen this before).
'over' becomes 'o' (cricket abbreviation**).
'line' becomes 'l'.
(Other definitions for tombola that I've seen before include "Lottery game", "Lottery with a rotating drum", "Draw", "Ticket-drawing game", "Tickets are drawn from a revolving drum in this raffle", "form", "fair game", "fund-raising event", "gambling activity" and "Form of lottery".)