Mailman on board? (6)

I believe the answer is:

knight

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But I can't explain it at all! Could you help me understand?

(Other definitions for knight that I've seen before include "Chess piece shaped like hors's head" , "cavalier" , "Chess piece shaped like horse's head" , "Gawain?" , "Honour" .)

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

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