Goat initially beat back bloodsucker (4)
I believe the answer is:
gnat
'bloodsucker' is the definition.
(I've seen this before)
'goat initially beat back' is the wordplay.
'initially' suggests taking the first letters.
'beat' becomes 'tan' (I've seen this before).
'back' shows that the letters should be reversed in order.
The first letter of 'goat' is 'g'.
'tan' written backwards gives 'nat'.
'g'+'nat'='GNAT'
(Other definitions for gnat that I've seen before include "Two-winged fly" , "Small biting fly" , "Something that bites" , "Small biting flying insect" , "Small (swarming) fly" .)