Bad lad with mere stone (7)
I believe the answer is:
emerald
'stone' is the definition.
(type of gemstone)
'bad lad with mere' is the wordplay.
'bad' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'lad' put after 'mere' is 'merelad'.
'merelad' anagrammed gives 'EMERALD'.
(Other definitions for emerald that I've seen before include "Precious gemstone" , "Ireland is often called The .......Isle" , "On of the Greens" , "Gem associated with Ireland" , "Green gemstone" .)