To breathe a little is quite usual! (5)
I believe the answer is:
habit
'quite usual' is the definition.
I can't judge whether this defines the answer.
'to breathe a little' is the wordplay.
'to breathe' becomes 'ha' (I can't explain this - if you can you should give a lot more credence to this answer).
'a little' becomes 'bit' (both can mean a small amount of something).
'ha'+'bit'='HABIT'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "can cause addiction" , "Custom or established practice" , "nun gets into" , "Monk's gown or custom" , "Ingrained behaviour" .)