Helsing finally beat up bloodsucker (4)
I believe the answer is:
gnat
'bloodsucker' is the definition.
(I've seen this before)
'helsing finally beat up' is the wordplay.
'finally' says to take the final letters.
'beat' becomes 'tan' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
The last letter of 'helsing' is 'g'.
'tan' reversed gives 'nat'.
'g'+'nat'='GNAT'
(Other definitions for gnat that I've seen before include "Tang (anag.)" , "Small biting fly" , "Insect often hovering over stagnant water" , "Small, swarming insect" , "Any of various small biting flies" .)