The number on this is twelve (4)
I believe the answer is:
noon
'is twelve' is the definition.
(twelve noon)
'the number on' is the wordplay.
'the number' becomes 'no' (abbreviation for 'number').
'no'+'on'='NOON'
'this' is the link.
(Other definitions for noon that I've seen before include "Twelve during daylight" , "Twelve o'clock in the day" , "12 pm" , "High time of day?" , "Twelve o'clock, midday" .)