Littlest in a litter (4)
I believe the answer is:
runt
'littlest in 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 "Weakling of the litter" , "Smallest squealer" , "little piggy" , "Undersized pig" , "Weak little squealer" .)