Upright supporters of old-fashioned sea travel (5)

I believe the answer is:

masts

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'upright supporters' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.

I can't explain the rest of the clue.

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(Other definitions for masts that I've seen before include "Eg, transmitter towers" , "things for flags" .)

I've seen this clue in the Evening Standard.

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