Weakest of the litter (4)
I believe the answer is:
runt
'weakest of the litter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for runt that I've seen before include "Smallest piglet of a litter" , "little piggy" , "Undersized pig" , "Weakling of the litter" , "Turn into smallest of the litter" .)