Enthusiastic for knee operation (4)

I believe the answer is:

keen

I'm a little stuck... Click here to teach me more about this clue!

'enthusiastic' is the definition.
(I've seen this before)

'knee operation' is the wordplay.
'knee' anagrams to 'KEEN'.
However, I am not sure how the anagram is indicated.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for keen that I've seen before include "moan" , "Razor-edged" , "Diligent" , "Agog" , "Avid, enthusiastic" .)

I've seen this clue in The Mirror.

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