Pirate as poor becomes sponger (8)
I believe the answer is:
parasite
'sponger' is the definition.
(I know that sponger can be written as parasite)
'pirate as poor' is the wordplay.
'poor' is an anagram indicator.
'pirate'+'as'='pirateas'
'pirateas' is an anagram of 'PARASITE'.
'becomes' acts as a link.
(Other definitions for parasite that I've seen before include "Organism growing on or in another" , "It depends on the host" , "unwanted guest" , "It lives off its host" , "One living off another" .)