Game allowed to block road (8)
I believe the answer is:
roulette
'game' is the definition.
(roulette is a kind of game)
'allowed to block road' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'block' indicates putting letters inside (upper walls off glass inside formal).
'road' becomes 'route' (synonyms).
'let' placed into '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" .)