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' says the answer is hidden in the clue.
'GEM' is hidden within '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?" .)