Game allowed to block road (8)
I believe the answer is:
roulette
'game' is the definition.
(I know that roulette is a type of gambling game)
'allowed to block road' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'block' is an insertion indicator (upper walls off glass inside formal).
'road' becomes 'route' (synonyms).
'let' inserted within 'route' is 'ROULETTE'.
(Other definitions for roulette that I've seen before include "risky venture" , "it's for the better, supposedly" , "Half of 1D [GAME OR BIRD]" , "it's a gamble" , "Spinning-wheel game" .)