Exposure when very warm in river (5)
I believe the answer is:
photo
'exposure' is the definition.
('photo' can be a synonym of 'exposure')
'very warm in river' is the wordplay.
'very warm' becomes 'hot' (I've seen this before).
'in' indicates putting letters inside.
'river' becomes 'po' (Italian river).
'hot' going into 'po' is 'PHOTO'.
'when' is the link.
(Other definitions for photo that I've seen before include "Snap (abbrev.)" , "Snap, exposure, in short" , "Snap, picture" , "Snap - shot" , "Pic (abbr)" .)