Game allowed to block path (8)
I believe the answer is:
roulette
'game' is the definition.
(I know that roulette is a type of gambling game)
'allowed to block path' 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).
'path' becomes 'route' (synonyms).
'let' going within 'route' is 'ROULETTE'.
(Other definitions for roulette that I've seen before include "SPORT/GAME" , "it's for the better, supposedly" , "Gambling device" , "Form of gambling" , "Gambling game with spinning wheel and ball" .)