Construction you once placed in river (8)
I believe the answer is:
outhouse
'construction' is the definition.
(outhouse is a kind of construction)
'you once placed in river' is the wordplay.
'you once' becomes 'thou'.
'placed in' indicates putting letters inside (I've seen this in other clues).
'river' becomes 'ouse' (Yorkshire river).
'thou' going within 'ouse' is 'OUTHOUSE'.
(Other definitions for outhouse that I've seen before include "Building near home" , "Building in the grounds of a house" , "Shed or barn belonging to house but not part of it" , "Smaller building separate from main one" , "External building" .)