Green mould originally occupying busy leader (7)
I believe the answer is:
emerald
'green' is the definition.
(emerald is a shade of green)
'mould originally occupying busy leader' is the wordplay.
'originally' suggests taking the first letters.
'occupying' is an insertion indicator (inserted letters occupy or enter the word).
'busy' is an anagram indicator.
The initial letter of 'mould' is 'm'.
'leader' anagrammed gives 'eerald'.
'm' put inside 'eerald' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "On of the Greens" , "Green gemstone" , "Precious gemstone" , "Gem; type of moth" , "La couleur" .)