River rising -- it starts to inflate old nun's habit (9)
I believe the answer is:
tradition
'habit' is the definition.
(I have seen 'old habit' mean 'tradition' so perhaps 'habit' could also mean 'tradition')
'river rising it starts to inflate old nun's' is the wordplay.
'river' becomes 'dart' (I've seen this before**).
'rising' says the letters should be written in reverse (going up in a down clue).
'starts to' indicates taking the first letters.
The first letters of 'inflate old nuns' is 'ion'.
'dart' written backwards gives 'trad'.
'trad'+'it'+'ion'='TRADITION'
(Other definitions for tradition that I've seen before include "Body of beliefs and customs handed down through generations" , "Inherited custom" , "Established custom" , "Custom passed on" , "Custom, long-standing practice" .)