Smallest pup (4)
I believe the answer is:
runt
'smallest pup' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for runt that I've seen before include "Littlest oinker" , "Turn into smallest of the litter" , "Weakest animal in a litter" , "Weak individual" , "Weakling of the litter" .)