Toilet possibly? Previously you needed to go in river (8)
I believe the answer is:
outhouse
'toilet possibly?' is the definition.
(outhouse is a kind of toilet)
'previously you needed to go in river' is the wordplay.
'previously you' becomes 'thou' (historical form of 'you').
'needed to go in' indicates putting letters inside.
'river' becomes 'ouse' (Yorkshire river).
'thou' put into 'ouse' is 'OUTHOUSE'.
(Other definitions for outhouse that I've seen before include "Building in the grounds of a house" , "Building near home" , "Smaller building separate from main one" , "External building" , "Shed or barn belonging to house but not part of it" .)