It's ordinary practice for some guys to be wise (5)
I believe the answer is:
usage
'it's ordinary practice for some guys' is the definition.
I can't judge whether this definition defines the answer.
'be wise' is the wordplay.
'be' becomes 'u' (I can't explain this - if you can you should give a lot more credence to this answer).
'wise' becomes 'sage' (similar in meaning).
'u'+'sage'='USAGE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for usage that I've seen before include "Utilisation" , "etiquette" , "Customary way in which language is spoken or written" , "Habit" , "Particular meaning of a word" .)