Rats! Game’s spoilt by big celebrity (8)
I believe the answer is:
megastar
'big celebrity' is the definition.
'rats game's spoilt' is the wordplay.
'spoilt' indicates anagramming the letters.
'rats'+'game'='ratsgame'
'ratsgame' anagrammed gives 'MEGASTAR'.
'by' acts as a link.
(Other definitions for megastar that I've seen before include "Great Sam (anag)" , "A-list celeb" , "Very famous artist" , "Big personality" , "Huge celebrity" .)