Unacceptable and preposterous fashion number (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
'preposterous fashion number' is the wordplay.
I cannot really understand how this works, but
'fashion' could be 'ton' (I've seen this before) and 'ton' is present in the answer.
'number' could be 'no' (abbreviation) and 'no' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'and' acts as a link.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Modern phrase for unacceptable" , "Informally, can't be accepted or tolerated" , "taboo" , "Out of the question" , "Definitely inadvisable" .)