It's habitual for us to be on time (5)
I believe the answer is:
usage
'it's habitual' is the definition.
I can't tell whether this definition defines the answer.
'us to be on time' is the wordplay.
'to be on' is a charade indicator (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 "Customary practice" , "Usual practice" , "Way of treating" , "Tradition - rate of consumption" , "Interest of old" .)