Star trio cast as double-crossers (8)
I believe the answer is:
traitors
'double-crossers' is the definition.
('traitor' can be a synonym of 'double-crosser')
'star trio cast' is the wordplay.
'cast' indicates an anagram (letters are dispersed or cast around).
'star'+'trio'='startrio'
'startrio' anagrammed gives 'TRAITORS'.
'as' is the link.
(Other definitions for traitors that I've seen before include "They betray causes and countries" , "Bad eggs" , "Quislings, collaborators 98)" , "Double-dealers" .)