Duty shown by former monarch disembarking from train (6)
I believe the answer is:
excise
'duty' is the definition.
(both can mean a tax)
'former monarch disembarking from train' is the wordplay.
'former monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'disembarking from' is a deletion indicator.
'train' becomes 'exercise' (I've seen this before).
'exercise' with 'er' taken away is 'EXCISE'.
'shown by' acts as a link.
(Other definitions for excise that I've seen before include "Type of tax" , "Remove by cutting out" , "Internal tax" , "Tax on certain goods and commodities" , "Cut out; commodity tax" .)