Red dice used in important game (7)
I believe the answer is:
decider
'important game' is the definition.
(I've seen this before)
'red dice used' is the wordplay.
'used' indicates anagramming the letters.
'red'+'dice'='reddice'
'reddice' anagrammed gives 'DECIDER'.
'in' acts as a link.
(Other definitions for decider that I've seen before include "Game settling a contest" , "Last game after a draw" , "Ultimate goal" , "Important match" , "It settles the matter" .)