Curious thing on river? It's what the retiring may assume (9)
I believe the answer is:
nightwear
'what the retiring may assume' is the definition.
(retire can mean to go to bed)
'curious thing on river?' is the wordplay.
'curious' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'river?' becomes 'wear' (river in the north east of England).
'thing' with letters rearranged gives 'night'.
'night'+'wear'='NIGHTWEAR'
'it's' is the link.
(Other definitions for nightwear that I've seen before include "this brings bed to mind?" , "Pyjamas e.g." , "Clobber after hours" , "evening dress" , "Certain garments" .)