... and back, into a Mitford-approved vein to the point of producing disgust? (2,7)
I believe the answer is:
ad nauseam
'the point of producing disgust?' is the definition.
I can't judge whether this defines the answer.
'and back into a mitford-approved vein' is the wordplay.
'back' shows that the letters should be reversed in order.
'into' means one lot of letters goes inside another.
'a mitford-approved' becomes 'au' (I can't explain this - if you can you should give a lot more credence to this answer).
'vein' becomes 'seam' (I've seen this in another clue).
'and' backwards is 'dna'.
'dna' placed within 'au' is 'adnau'.
'adnau'+'seam'='AD NAUSEAM'
'to' is the link.
Can you help me to learn more?
(Other definitions for ad nauseam that I've seen before include "To a disgusting extent" , "Done so often that it becomes tiresome" , "To the point of disgust" , "Interminably" , "to an excessive extent" .)