Test beagle's arrangement for insect (4,6)
I believe the answer is:
stag beetle
'insect' is the definition.
(stag beetle is a kind of insect)
'test beagle's arrangement' is the wordplay.
'arrangement' indicates anagramming the letters.
'test'+'beagle'='testbeagle'
'testbeagle' with letters rearranged gives 'STAG-BEETLE'.
'for' is the link.
(Other definitions for stag beetle that I've seen before include "Six-footer" , "something some folk find creepy" , "Insect with antler-like jaws" , "Large insect with branched mandibles" , "Horny little creature" .)