Large insect (4,6)
'large insect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stag beetle that I've seen before include "something some folk find creepy" , "Horny little creature" , "Insect with antler-like jaws" , "Large insect with branched mandibles" , "Six-footer" .)
(Another definition for horsefly that I've seen is " sucker".)