Avid to get knee fixed (4)
I believe the answer is:
keen
'avid' is the definition.
(I've seen this before)
'knee fixed' is the wordplay.
'fixed' indicates anagramming the letters.
'knee' with letters rearranged gives 'KEEN'.
'to get' is the link.
(Other definitions for keen that I've seen before include "Itching" , "Eager; wail in grief" , "Agog" , "Piercing" , "Eager; dirge" .)