Shed when you go into the river (8)
I believe the answer is:
outhouse
'shed' is the definition.
(I've seen this before)
'you go into the river' is the wordplay.
'you' becomes 'thou' (I've seen this in other clues).
'go into' means one lot of letters goes inside another (some letters go into others).
'the river' becomes 'ouse' (Yorkshire river).
'thou' inserted inside 'ouse' is 'OUTHOUSE'.
'when' is the link.
(Other definitions for outhouse that I've seen before include "Construction" , "External building" , "Building near home" , "Building in the grounds of a house" , "Smaller building separate from main one" .)