Smallest of the litter (4)
I believe the answer is:
runt
'smallest of the 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 piglet of a litter" , "Litter-weakling" , "Littlest oinker" , "stunted figure" , "Smallest animal in a litter" .)