Sin permitted? Not at first! (5)
I believe the answer is:
greed
'sin' is the definition.
(I know that greed is a type of deadly sin)
'permitted? not at first' is the wordplay.
'permitted?' becomes 'agreed'.
'not at first' means to remove the first letter.
'agreed' with its first letter taken off is 'GREED'.
(Other definitions for greed that I've seen before include "Rapacity" , "Avarice, cupidity" , "Selfish desire" , "Covetousness" , "Gluttony" .)