Huge upset for one of the Stones? (4)
I believe the answer is:
mega
'huge' is the definition.
(mega can mean very large)
'upset for one of the stones?' is the wordplay.
I cannot quite see how this works, but
'one' could be 'me' (a setter might use 'one' to humourously mean 'me') and 'me' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mega that I've seen before include "ace" , "extremely good" , "Fantastic!" , "Game (anag) -- great" , "Very large (colloq.)" .)