A sober habit (10)
I believe the answer is:
temperance
'a sober habit' is the definition.
(the habit of being sober)
'a sober' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for temperance that I've seen before include "Moderation" , "Avoidance of extremes" , "for staying off drink" , "Abstinence from alcohol" , "Feature of traditional Methodism" .)