Duke has finished the port (5)
I believe the answer is:
dover
'the port' is the definition.
(I've seen this before)
'duke has finished' is the wordplay.
'duke' becomes 'd'.
'has' is a charade indicator (letters next to each other).
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'd'+'over'='DOVER'
(Other definitions for dover that I've seen before include "There'll be blue clouds over the white cliffs here" , "Sort of sole" , "Kentish port" , "Be aware of" , "White cliffs port" .)