Creepy-crawly’s nicest cooked (6)
I believe the answer is:
insect
'creepy-crawly's' is the definition.
(insect is a kind of invertebrate)
'nicest cooked' is the wordplay.
'cooked' indicates an anagram.
'nicest' anagrammed gives 'INSECT'.
(Other definitions for insect that I've seen before include "Soldier perhaps" , "cricket, maybe" , "Small six-legged creature" , "Small air-breathing arthropod" , "Possibly fly" .)