No drink? That's upset a monster (6)
I believe the answer is:
gorgon
'a monster' is the definition.
(Gorgon is an example)
'no drink? that's upset' is the wordplay.
'drink?' becomes 'grog' (grog is a kind of drink).
'that's upset' says the letters should be written backwards.
'no'+'grog'='nogrog'
'nogrog' reversed gives 'GORGON'.
(Other definitions for gorgon that I've seen before include "Medusa such a monster" , "Mythical sister with snakes for hair" , "Monster, petrifying glance" , "Either Stheno, Euryale or Medusa" , "She had the power to turn people to stone" .)