Bloodsucker in punch-up? (4)
I believe the answer is:
gnat
'bloodsucker' is the definition.
(I've seen this before)
'punch-up?' is the wordplay.
'punch' becomes 't'ang' (I've seen this in another clue).
'up?' says the letters should be written in reverse (in down clue: letters go upwards).
'tang' back-to-front is 'GNAT'.
'in' acts as a link.
(Other definitions for gnat that I've seen before include "Something with bite" , "Small (swarming) fly" , "Tang of a biting fly" , "mozzie" , "Small, swarming insect" .)