Out of sorts, hit catching back of knee: hit back! (5,3)
I believe the answer is:
below par
'out of sorts' is the definition.
(I've seen this before)
'hit catching back of knee hit back' is the wordplay.
I cannot really understand how this works, but
'hit' could be 'par' (parring is a kind of hitting) and 'par' is found within the answer.
The remaining letters 'below' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for below par that I've seen before include "Not in full health" , "Not up to usual standard, like golfer" , "Worse than usual" , "Out of sorts" , "Worse than expected" .)