Smallest of a litter (4)
I believe the answer is:
runt
'smallest of a litter' 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 "Smallest pig in litter" , "Weak individual" , "Litter-weakling" , "Smallest piglet of a litter" , "little piggy" .)