Breathe a little, as is the custom (5)
I believe the answer is:
habit
'the custom' is the definition.
(habit is a kind of custom)
'breathe a little as' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'little' could be 'bit' (I've seen this in another clue) and 'bit' is located in the answer.
A single letter 'h' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Custom or established practice" , "Usual practice; garment" , "Monk's garb" , "Monk's gown or custom" , "It's worn in order" .)