Duke finished port (5)
I believe the answer is:
dover
'port' is the definition.
(I've seen this before)
'duke finished' is the wordplay.
'duke' becomes 'd'.
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'd'+'over'='DOVER'
(Other definitions for dover that I've seen before include "UK shipping forecast area" , "fish" , "White cliffs port" , "Kent ferry port" , "Channel port" .)