Thrush biting insect with its head down (5)

I believe the answer is:

ousel

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'thrush' is the definition.
(ousel is a kind of thrush)

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

(Other definitions for ousel that I've seen before include "The bird like a blackbird from Seoul" , "Bird similar to blackbird" , "Songbird inhabiting fast-flowing streams" , "Former name for blackbird" , "Type of thrush" .)

I've seen this clue in The Guardian.

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