Envious, mostly a key sin (5)
I believe the answer is:
greed
'sin' is the definition.
(I know that greed is a type of deadly sin)
'envious mostly a key' is the wordplay.
'envious' becomes 'green'.
'mostly' means to remove the last letter (most but not all of the word).
'a key' becomes 'd' (musical key).
'green' with its final letter taken off is 'gree'.
'gree'+'d'='GREED'
(Other definitions for greed that I've seen before include "Wanting too much" , "Circuit" , "Covetousness" , "Avarice, cupidity" , "Selfish desire" .)