Sucker cast as pirate (8)
I believe the answer is:
parasite
'sucker' is the definition.
(thesaurus)
'cast as pirate' is the wordplay.
'cast' indicates an anagram.
'as'+'pirate'='aspirate'
'aspirate' with letters rearranged gives 'PARASITE'.
(Other definitions for parasite that I've seen before include "sponge" , "It depends on the host" , "Twenty five dn [LEECH]" , "Self-interested hanger-on" , "It lives off its host" .)