If still in bed, it means you can't return the ball (3,2)
I believe the answer is:
not up
'can't return the ball' is the definition.
I know nothing about this answer so I can't tell whether this works.
'if still in bed it means you' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
'you' could be 'u' (homophones) and 'u' is found within the answer.
The remaining letters 'nop' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
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" .)