Unacceptable and rank, but exceptional (11)
I believe the answer is:
outstanding
'exceptional' is the definition.
(thesaurus)
'unacceptable and rank' is the wordplay.
'unacceptable' becomes 'out' (out can mean incorrect or inappropriate).
'and' is a charade indicator (letters next to each other).
'rank' becomes 'standing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'out'+'standing'='OUTSTANDING'
'but' is the link.
Can you help me to learn more?
(Other definitions for outstanding that I've seen before include "Unpaid" , "unsettled" , "to be settled" , "superior" , "Not paid yet" .)