Goons initially beat back bloodsucker (4)
I believe the answer is:
gnat
'bloodsucker' is the definition.
(I've seen this before)
'goons initially beat back' is the wordplay.
'initially' indicates taking the first letters.
'beat' becomes 'tan' (tan can mean to beat or hit).
'back' shows that the letters should be reversed in order.
The first letter of 'goons' is 'g'.
'tan' reversed gives 'nat'.
'g'+'nat'='GNAT'
(Other definitions for gnat that I've seen before include "annoying flier" , "Small biting flying insect" , "Insect like small mosquito" , "Stinging insect" , "a little bit of a nuisance" .)