Gang at last beat up bloodsucker (4)
I believe the answer is:
gnat
'bloodsucker' is the definition.
(I've seen this before)
'gang at last beat up' is the wordplay.
'at last' indicates one should take the final letters.
'beat' becomes 'tan' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The last letter of 'gang' is 'g'.
'tan' written backwards gives 'nat'.
'g'+'nat'='GNAT'
(Other definitions for gnat that I've seen before include "a little bit of a nuisance" , "insect's" , "little irritation" , "Something with bite" , "Small bloodsucker" .)