Sweet tablet put in water after upset (8)
I believe the answer is:
lollipop
'sweet' is the definition.
(lollipop is a kind of sweet)
'tablet put in water after upset' is the wordplay.
'tablet' becomes 'pill' (synonyms).
'put in' means one lot of letters goes inside another.
'water' becomes 'pool' (pool is a kind of water).
'after upset' shows that the letters should be reversed in order (in a down clue, letters go up).
'pill' placed into 'pool' is 'popillol'.
'popillol' back-to-front is 'LOLLIPOP'.
(Other definitions for lollipop that I've seen before include "Nutcracker Suite, say" , "Popsicle" , "sucker" , "Sweet; popular work" , "Woman seeing children safely across the road" .)