One to unload clipper? (6)

I believe the answer is:

docker

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

I can't explain the remainder of the clue.

(Other definitions for docker that I've seen before include "Cargo handler" , "He loads and unloads ships" , "One unloading" , "But does one cut" , "One who loads ships" .)

I've seen this clue in The Sun.

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