A precious stone returned? That’s great! (4)
I believe the answer is:
mega
'great' is the definition.
(old-fashioned UK slang for 'very good')
'a precious stone returned?' is the wordplay.
'precious stone' becomes 'gem' (gem stone).
'returned?' shows that the letters should be reversed in order (I've seen 'return of' mean this).
'a'+'gem'='agem'
'agem' in reverse letter order is 'MEGA'.
'that's' acts as a link.
(Other definitions for mega that I've seen before include "Informally great or large" , "Very large (prefix)" , "ace" , "extremely good" , "Great American" .)