Conservative oyster-lease marker (5-2-3-3)

I believe the answer is:

stick-in-the-mud

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'conservative' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for stick-in-the-mud that I've seen before include "Old-fashioned type" , "Fogeyish" , "One who won't get moving" , "Old fashioned" , "Fuddy-duddy" .)

I've seen this clue in the Sydney Morning Herald.

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