Moor by river bank (4)

I believe the answer is:

tier

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'bank' is the definition.
(both can mean a row of similar things)

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

(Other definitions for tier that I've seen before include "Rank, level" , "Row - one forming a knot" , "Plague" , "Bank - row" , "'Level, grade (4)'" .)

I've seen this clue in The Independent and The Sun.

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