Star one rubbished in act of betrayal (7)
I believe the answer is:
treason
'act of betrayal' is the definition.
(I've seen this before)
'star one rubbished' is the wordplay.
'rubbished' indicates an anagram.
'star'+'one'='starone'
'starone' with letters rearranged gives 'TREASON'.
'in' acts as a link.
(Other definitions for treason that I've seen before include "Crime of treachery against the state" , "Serious crime" , "Betrayal (of one's country)" , "Perfidy" , "subversion" .)