A deal leads to a game (5)
I believe the answer is:
lotto
'a game' is the definition.
(the UK National Lottery)
'a deal leads to' is the wordplay.
'a deal' becomes 'lot' ('lot' can be a synonym of 'deal').
'leads' means one lot of letters go next to another (some letters go in front of others).
'lot'+'to'='LOTTO'
(Other definitions for lotto that I've seen before include "chancy business" , "Prize draw" , "Housey-housey, bingo" , "Game of chance similar to bingo" , "Venetian painter - gambling game" .)