Crossword Genius

A beach’s length? (6)

Ross

I think it could be either:

margin

strand

I believe this clue is a double definition.

'a beach's' is the first definition.
(thesaurus)

'length?' is the second definition.
(thesaurus)

(Other definitions for margin that I've seen before include "Edge or space for error" , "bank" , "Blank space around the edge" , "Permissible difference" , "Blank space around a page" .)

I've seen this clue in The Times and The Sunday Times.
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