Invertebrate creature (6)
I believe the answer is:
insect
'invertebrate creature' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for insect that I've seen before include "Bug" , "Small invertebrate arthropod" , "Creepy-crawly" , "Small creature with six legs and no backbone" , "Fly, say?" .)