It's customary for us to be on time (5)
I believe the answer is:
usage
'it's customary' is the definition.
I can't judge whether this definition defines the answer.
'us to be on time' is the wordplay.
'to be on' says to put letters next to each other.
'time' becomes 'age' (an age is a period of time).
'us'+'age'='USAGE'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for usage that I've seen before include "Treatment; tradition" , "Way of employing" , "management" , "Accepted or habitual practice" , "etiquette" .)