Gemstone - a dealer maybe holds a number (7)
I believe the answer is:
emerald
'gemstone' is the definition.
(I know that emerald is a type of transparent gem)
'a dealer maybe holds a number' is the wordplay.
'maybe' indicates anagramming the letters.
'holds' indicates putting letters inside.
'a number' becomes 'm' (M is an example).
'dealer' is an anagram of 'eerald'.
'eerald' enclosing 'm' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "and 13 Across: Poetic name for Ireland" , "city in Oz?" , "Green gem" , "Mere lad (anag.)" , "Shade of green" .)