Twenty-four hours? Not a bit of it! (3,3)
I believe the answer is:
all day
'bit of it' is the definition.
The definition suggests an adjective which matches the answer.
'twenty-four hours? not a' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'twenty-four hours?' could be 'day' ('day' can be a synonym of 'twenty-four hours') and 'day' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for all day that I've seen before include "until even" , "From dawn to dusk" , "morning and afternoon" .)