Nicest form of invertebrate (6)
I believe the answer is:
insect
'invertebrate' is the definition.
(I know that insect is a type of arthropod)
'nicest form' is the wordplay.
'form' indicates anagramming the letters (another form of the letters).
'nicest' is an anagram of 'INSECT'.
'of' is the link.
(Other definitions for insect that I've seen before include "cricket, say" , "Praying mantis, say" , "Nicest (anag) - six-footer" , "Bee or beetle, for example" , "Small invertebrate creature" .)