Elbow possibly cut by stake not a sporting injury (7,6)
I believe the answer is:
joggers nipple
'sporting injury' is the definition.
(injury commonly affecting joggers)
'elbow possibly cut by stake not a' is the wordplay.
'elbow possibly' becomes 'jogger' (you might use your elbow to jog someone else).
'cut by' becomes 'snip' (snipping is a kind of cutting. I am not sure about the 'by' bit.).
'stake' becomes 'pale' (pale can mean a wooden stake or post).
'not' is a deletion indicator.
'pale' with 'a' taken out is 'ple'.
'jogger'+'snip'+'ple'='JOGGERS NIPPLE'
(Other definitions for joggers nipple that I've seen before include "Painful chest rubbing experienced by a runner" , "agonising result of friction, possibly?" .)