Weakest of the litter (4)
I believe the answer is:
runt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">weakest of the litter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)