Stone tossed back by a giant (4)
I believe the answer is:
mega
'giant' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'stone tossed back by a' is the wordplay.
'stone' becomes 'gem' (I've seen this before).
'tossed back' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'gem' backwards is 'meg'.
'meg'+'a'='MEGA'
Can you help me to learn more?
(Other definitions for mega that I've seen before include "Terrific" , "Big" , "wicked" , "Huge (slang)" , "extremely good" .)