PM no longer bluff, brooding antihero (10)
I believe the answer is:
heathcliff
'brooding antihero' is the definition.
(character in Emily Bronte's Wuthering Heights)
'pm no longer bluff' is the wordplay.
'pm no longer' becomes 'Heath' (prime minister Edward Heath).
'bluff' becomes 'cliff' (bluff can mean a cliff).
'heath'+'cliff'='HEATHCLIFF'
(Other definitions for heathcliff that I've seen before include "character in novel" , "tortured character" , "He loved Cathy Earnshaw" , "Hero of Wuthering Heights" .)