Line adopted in instinctive, fashionable sin (8)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

L in GUT (instinctive, as in gut reaction) TONY (fashionable, in North America, allegedly); interestingly, while TON meaning fashion pops up in crosswords regularly, TONY is a much rarer visitor. A lot to do with morphology and a little to do with tradition, I would imagine.