Using emerald to hide stone? (3)
I believe the answer is:
gem
'stone?' is the definition.
(I know that gem can be written as gem)
'using emerald to hide' is the wordplay.
'to hide' indicates a hidden word.
'GEM' can be found hidden inside 'using emerald'.
(Other definitions for gem that I've seen before include "Valued person; jewel" , "Half of 7 [SIGN OF THE ZODIAC]" , "A jewel or precious stone" , "Precious or semi-precious stone" , "perfect specimen" .)