Mere lad potentially a gem? (7)
I believe the answer is:
emerald
'a gem?' is the definition.
(type of gem stone)
'mere lad potentially' is the wordplay.
'potentially' is an anagram indicator (letters have the potential to spell a new word).
'mere'+'lad'='merelad'
'merelad' anagrammed gives 'EMERALD'.
(Other definitions for emerald that I've seen before include "city in Oz?" , "Small size of type, in printing" , "and 13 Across: Poetic name for Ireland" , "Highly-valued variety of beryl" , "Shade of green" .)