Partner no longer certain about river outing (8)
I believe the answer is:
exposure
'outing' is the definition.
(to expose someone is to out them)
'partner no longer certain about river' is the wordplay.
'partner no longer' becomes 'ex' (I've seen this in other clues).
'certain' becomes 'sure' (similar in meaning).
'about' says to put letters next to each other.
'river' becomes 'po' (Italian river).
'sure' after 'po' is 'posure'.
'ex'+'posure'='EXPOSURE'
(Other definitions for exposure that I've seen before include "The harmful effects on the body of extreme cold" , "Vulnerability; photograph" , "Unveiling" , "O pure sex! (anag.)" , "Revelation (of something secret)" .)