Eager for knee operation (4)
I believe the answer is:
keen
'eager' is the definition.
(I've seen this before)
'knee operation' is the wordplay.
'knee' can be anagrammed to 'KEEN'.
Though, I'm not sure how the anagram is indicated.
'for' is the link.
Can you help me to learn more?
(Other definitions for keen that I've seen before include "Having a fine edge to weep over the dead" , "Sharp -- express grief" , "Anxious" , "Avid, enthusiastic" , "Eager, enthusiastic" .)