Impostor’s bribe rejected by head of state (5)
I believe the answer is:
poser
'impostor's' is the definition.
(thesaurus)
'bribe rejected by head of state' is the wordplay.
'bribe' becomes 'sop' (sop can mean a concession or bribe).
'rejected' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'head of state' becomes 'ER' (abbreviation for Elizabeth Regina).
'sop' reversed gives 'pos'.
'pos'+'er'='POSER'
(Other definitions for poser that I've seen before include "Puzzling question about ropes" , "One pretends to be something they are not" , "Question - a sitter" , "Ropes (anag.)" , "subject for an artist?" .)