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Be appropriate, like a wagtail by falling water but not river (7)

Ross

I believe the answer is:

pertain

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'be appropriate' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

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(Other definitions for pertain that I've seen before include "Relate (to), apply (to)" , "Painter inclined to be relevant" , "Painter (anag) - relate (to)" , "Refer" , "Be relevant, or belong as part" .)

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