A number finished the port (5)
I believe the answer is:
dover
'the port' is the definition.
(I've seen this before)
'a number finished' is the wordplay.
'a number' becomes 'd' (D is an example**).
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'd'+'over'='DOVER'
(Other definitions for dover that I've seen before include "fish" , "The largest English ferry port" , "Be aware of" , "Cinque Port" , "UK shipping forecast area" .)