I'm given shore duty (6)

I believe the answer is:

impost

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'duty' is the definition.
(I know that impost is a type of duty)

'i'm given shore' is the wordplay.
'given shore' becomes 'post' (I can't justify this - if you can you should believe this answer much more).
'im'+'post'='IMPOST'

Can you help me to learn more?

(Other definitions for impost that I've seen before include "A tax or duty levied" , "Customs duty" .)

I've seen this clue in The Guardian and The Globe and Mail.

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