Certainly not working when it's lunchtime? (4)
I believe the answer is:
noon
'when it's lunchtime?' is the definition.
(12 noon is often lunchtime)
'certainly not working' is the wordplay.
'certainly not' becomes 'no'.
'working' becomes 'on' (on can mean working or activated).
'no'+'on'='NOON'
(Other definitions for noon that I've seen before include "Twelve o'clock, midday" , "The middle of the day" , "See 8" , "12 o'clock" , "[LUNCHTIME] ?" .)