Could be the nicest creature (6)
I believe the answer is:
insect
'creature' is the definition.
(insect is a kind of creature)
'could be the nicest' is the wordplay.
'could be' indicates anagramming the letters.
'nicest' is an anagram of 'INSECT'.
(Other definitions for insect that I've seen before include "Small arthropod creature" , "Eg, beetle or bee" , "Small animal with six legs" , "Perhaps nit" , "cricket, say" .)