Bird: do as might a happy harrier? (7)

I believe the answer is:

wagtail

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'bird' is the definition.
(wagtail is a kind of bird)

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

(Other definitions for wagtail that I've seen before include "Slender songbird - a law git (anag)" , "Small bird (pied/yellow)" , "singer" , "Common bird" , "Songbird - gala wit (anag)" .)

I've seen this clue in The Guardian.

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