One knocking forest spirits? (5,10)

I believe the answer is:

green woodpecker

'one knocking' is the definition.
(bird which knocks against trees)

'forest spirits?' is the wordplay.
'forest' becomes 'greenwood' (greenwood is a kind of forest).
'spirits?' becomes 'pecker' (I've seen this before).
'greenwood'+'pecker'='GREEN WOODPECKER'

(Another definition for green woodpecker that I've seen is " flyer".)

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

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