mostly clean. Thomas, one of the footmen, is gay, and most of the characters are accepting of it, especially in episode 6 of season 3, when they all say that 'he can't help the way he was born', etc. etc. Lord Grantham has a passing affair with a household servant who quickly leaves after said 'affair' (his lust for her is implied with him trying to convince her to sleep with him); Grantham quickly realizes his wrongdoing and is sorry for ever having gone down that road with a woman other than his wife. Also, in season 1, one of the daughters has a one night stand with a visiting guest and the guest ends up dying in her bed; all throughout the season she is remorseful (though I haven't quite figured out if it was just because it ruined her reputation or if she was truly sorry for the immoral act of it).
But yes, if I had to sum it up, I would say it's clean. It's certainly cleaner than 99% of the stuff on tv right now.