The number going to and fro is twelve, exactly (4)
I believe the answer is:
noon
'and fro is twelve exactly' is the definition.
I can't tell whether this definition defines the answer.
'the number going' is the wordplay.
'the number' becomes 'no' (abbreviation).
'going' becomes 'on' (both can mean functioning or activated).
'no'+'on'='NOON'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for noon that I've seen before include "12 o'clock" , "A moment of time" , "See 8" , "when only mad dogs go out?" , "Twelve during daylight" .)