Big fellows I beat in the backstreet (6)
I believe the answer is:
titans
'big fellows' is the definition.
(I've seen this before)
'i beat in the backstreet' is the wordplay.
'beat' becomes 'tan' (tan can mean to beat or hit).
'in the' is an insertion indicator.
'backstreet' becomes 'ts' ('st' backwards).
'i'+'tan'='itan'
'itan' placed into 'ts' is 'TITANS'.
(Other definitions for titans that I've seen before include "Elder deities of Greek myth" , "Taints in mythical giants" , "Giant gods of Greek mythology" , "Old Greek gods, very strong like Atlas" , "Mythological beings of great size and strength" .)