The smallest one’s somewhat disgruntled (4)
I believe the answer is:
runt
'the smallest one's' is the definition.
(the runt of the litter)
'somewhat disgruntled' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'RUNT' is hidden within 'disgruntled'.
(Other definitions for runt that I've seen before include "Turn into smallest of the litter" , "little one" , "little piggy" , "Weakest animal in a litter" , "Smallest animal in a litter" .)