Force to serve port (8)

I believe the answer is:

shanghai

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'port' is the definition.
(I know that Shanghai is a port)

I cannot understand how the remainder of the clue works.

(Other definitions for shanghai that I've seen before include "Kidnap - Chinese port" , "Take to sea" , "China's largest city" , "Trick (someone) into becoming a sailor" , "Press men at one time" .)

I've seen this clue in The Times.

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