Massive precious stone lifted by adult (4)
I believe the answer is:
mega
'massive' is the definition.
(mega can mean large)
'precious stone lifted by adult' is the wordplay.
'precious stone' becomes 'gem' (gem stone).
'lifted' shows that the letters should be reversed in order (going up in down clue).
'by' means one lot of letters go next to another.
'adult' becomes 'a' (abbreviation).
'gem' reversed gives 'meg'.
'meg'+'a'='MEGA'
(Other definitions for mega that I've seen before include "Huge (slang)" , "Game (anag) - excellent" , "Very large (prefix)" , "Fantastic!" , "ace" .)