Manufacture pork pies for one entering river port (5)
I believe the answer is:
liege
'manufacture pork pies' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'for one entering river port' is the wordplay.
'for one' becomes 'eg' (I've seen this in other clues).
'entering' indicates putting letters inside.
'river port' becomes 'lie' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'eg' put into 'lie' is 'LIEGE'.
Can you help me to learn more?
(Other definitions for liege that I've seen before include "Chieftain" , "Belgian city or feudal chief" , "Vassal, subject" , "Belgian, French-speaking city" , "Belgian city or feudal superior" .)