Some harbour dues with which Pakistanis are conversant (4)

I believe the answer is:

urdu

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'pakistanis are conversant' is the definition.
I can't tell whether this defines the answer.

'some harbour dues with' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'URDU' is hidden within 'harbour dues with'.

'which' is the link.

Can you help me to learn more?

(Other definitions for urdu that I've seen before include "speech" , "Language akin to Hindi" , "Pakistani language" , "Indian language" , "Channel to north of Isle of Wight" .)

I've seen this clue in The Irish Independent.

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