Tablet after start of stomach upset (5)
I believe the answer is:
spill
'upset' is the definition.
(I've seen this in another clue)
'tablet after start of stomach' is the wordplay.
'tablet' becomes 'pill' (synonyms).
'after' is a charade indicator (letters next to each other).
'start of' suggests taking the first letters.
The initial letter of 'stomach' is 's'.
'pill' after 's' is 'SPILL'.
(Other definitions for spill that I've seen before include "Wooden taper; discharge" , "Allow to drop - fire lighter" , "Cause to fall from horse" , "Overturn" , "Thin piece of wood used to light a fire" .)