Litter critter (4)
I believe the answer is:
runt
'litter critter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for runt that I've seen before include "Turn into smallest of the litter" , "Weakling of the litter" , "Litter-weakling" , "Weak little squealer" , "Weakest animal in a litter" .)