Knee pain? Oddly, it's the cap! (4)
I believe the answer is:
kepi
'the cap' is the definition.
(kepi is a kind of cap)
'knee pain? oddly' is the wordplay.
'oddly' means one should take alternating letters.
'knee'+'pain'='kneepain'
The alternating letters of 'kneepain' are 'KEPI'.
'it's' acts as a link.
(Other definitions for kepi that I've seen before include "French military hat" , "French army cap" , "French military cap" , "Military cap with circular top" , "Service cap" .)