Littlest of a litter (4)
I believe the answer is:
runt
'littlest 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 "little piggy" , "Weak little squealer" , "Litter-weakling" , "Littlest oinker" , "Turn into smallest of the litter" .)