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