Leg beast broke tackling alien insect (4,6)
I believe the answer is:
stag beetle
'insect' is the definition.
(stag beetle is a kind of insect)
'leg beast broke tackling alien' is the wordplay.
'broke' is an anagram indicator.
'tackling' is an insertion indicator (I've seen this in other clues).
'alien' becomes 'et' (extra terrestrial).
'leg'+'beast'='legbeast'
'legbeast' anagrammed gives 'stagbele'.
'stagbele' enclosing 'et' is 'STAG-BEETLE'.
(Other definitions for stag beetle that I've seen before include "Insect with antler-like jaws" , "something some folk find creepy" , "Six-footer" , "Large insect with branched mandibles" , "Horny little creature" .)