Untidy little pigs! (6)
I believe the answer is:
'untidy' is the definition.
Although both the answer and definition are adjectives, they are not in the same form of the adjective.
'little pigs?' is the wordplay.
'little' means to remove the last letter (the word is not quite complete).
'pigs?' becomes 'litters' (pigging is a kind of littering).
'litters' with its final letter taken away is 'LITTER'.
Can you help me to learn more?
(Other definitions for litter that I've seen before include "Young dogs, perhaps" , "that you may see in pigsty?" , "untidiness" , "Offspring - rubbish" , "cats go here" .)