Number working, twelve (4)
I believe the answer is:
noon
'twelve' is the definition.
(twelve noon)
'number working' is the wordplay.
'number' becomes 'no' (abbreviation).
'working' becomes 'on' (on can mean working or activated).
'no'+'on'='NOON'
(Other definitions for noon that I've seen before include "12 o'clock" , "High time of day?" , "The middle of the day" , "Twelve o'clock in the day" , "when only mad dogs go out?" .)