Smallest animal in a litter (4)
I believe the answer is:
runt
'smallest animal in a litter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for runt that I've seen before include "little one" , "Littlest oinker" , "Small pig" , "Smallest piglet of a litter" , "Undersized pig" .)