It’s found in mining emeralds (3)
I believe the answer is:
gem
'it's' is the definition.
I can't judge whether this definition defines the answer.
'in mining emeralds' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GEM' is hidden in the letters of 'mining emeralds'.
'found' is the link.
Can you help me to learn more?
(Other definitions for gem that I've seen before include "Valuable stone" , "Emerald or sapphire, say" , "36 Across or any other jewel" , "Diamond, ruby, or emerald" , "one of the Cairngorms?" .)