Line adopted in instinctive, fashionable sin (8)
I believe the answer is:
gluttony
'sin' is the definition.
(gluttony is a kind of sin)
'line adopted in instinctive fashionable' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'adopted in' indicates putting letters inside (inserted letters are welcomed or adopted).
'instinctive' becomes 'gut' (as in a gut feeling).
'fashionable' becomes 'tony' (I've seen this in another clue).
'l' inserted inside 'gut' is 'glut'.
'glut'+'tony'='GLUTTONY'
(Other definitions for gluttony that I've seen before include "Greed in eating" , "Habitual eating to excess" , "One of the seven deadly sins" , "bingeing" , "A deadly sin" .)