If still in bed, it means you can’t return the ball (3,2)
I believe the answer is:
not up
'if still in bed it means you can't' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'return the ball' is the wordplay.
'return' says the letters should be written in reverse.
'the ball' becomes 'puton' (I am not sure about this - if you are sure you should believe this answer much more).
'puton' in reverse letter order is 'NOT UP'.
Can you help me to learn more?
(Other definitions for not up that I've seen before include "still in bed" , "with tennis call" , "call in court" .)