Green's leader worried about source of material (7)
I believe the answer is:
emerald
'green's' is the definition.
(I've seen this before)
'leader worried about source of material' is the wordplay.
'worried' indicates anagramming the letters.
'about' indicates putting letters inside.
'source of' says to take the initial letters.
The first letter of 'material' is 'm'.
'leader' anagrammed gives 'eerald'.
'eerald' enclosing 'm' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "Green gemstone" , "Mere lad (anag.)" , "Small size of type, in printing" , "city in Oz?" , "Ireland is often called The .......Isle" .)