Not playing — indeterminate illness (4)
I believe the answer is:
ague
'illness' is the definition.
(I know that ague is a type of illness)
'not playing indeterminate' is the wordplay.
'not' indicates named letters should be taken away.
'playing' becomes 'v' ('playing' can mean 'versus' in sports).
'indeterminate' becomes 'vague' (I've seen this before).
'vague' with 'v' taken away is 'AGUE'.
(Other definitions for ague that I've seen before include "Malarial fever or fit of shivering" , "Feverish fit" , "Hot and cold fever" , "attack" , "Fit of the shakes" .)