25d Old philosophical doctrine uses sin to corrupt maiden (8). I've got the solution but it is such an archaism that none of my dictionaries (BRB, ODE, Collins) lists it at all. It's in the 1913 Webster's
Will37, it's in my Shorter Oxford English Dictionary (the two-volume one). I think the relative easiness of the clue was to make up for the relative obscurity of the answer!