Selling treasures maybe, duke is wearing ceremonial dress (13)
I believe the answer is:
disinvestment
'selling treasures maybe' is the definition.
I can't tell whether this definition defines the answer.
'duke is wearing ceremonial dress' is the wordplay.
'duke' becomes 'd' (abbreviation).
'wearing' becomes 'in' (in certain clothing).
'ceremonial dress' becomes 'vestment' (I can't justify this - if you can you should give a lot more credence to this answer).
'd'+'is'+'in'+'vestment'='DISINVESTMENT'
Can you help me to learn more?
(Other definitions for disinvestment that I've seen before include "Withdrawal from stocks, shares, etc" , "withdrawing capital" , "Reduction of assets" , "Withdrawal of, failure to replace, capital" .)