*Tablet (6,4)
'tablet' is the definition.
(synonyms)
This is all the clue.
(Other definitions for pill that I've seen before include "Pellet of medicine" , "Medicinal tablet" , "Form of medicine" , "nuisance to put up with" , "Medical tablet" .)
(Other definitions for bitter pill that I've seen before include "One unpleasant to swallow" , "It's hard to take" .)