Hammer raised while I finish off bloodsucker like a vampire (9)
I believe the answer is:
parasitic
'like a vampire' is the definition.
(a vampire is parasitic)
'hammer raised while i finish off bloodsucker' is the wordplay.
'hammer' becomes 'rap' (both can mean to hit).
'raised' says the letters should be written in reverse (going up in a down clue).
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'finish off' means to remove the last letter.
'bloodsucker' becomes 'tick' (blood-sucking parasite).
'rap' written backwards gives 'par'.
'tick' with its last letter taken away is 'tic'.
'par'+'as'+'i'+'tic'='PARASITIC'
(Other definitions for parasitic that I've seen before include "Living on another like flea" , "Living at the expense of the host organism" , "hanging on?" , "Bloodsucking" , "Having food provided by host" .)