River crosses northern valley (4)
I believe the answer is:
dene
'valley' is the definition.
(I've seen this before)
'river crosses northern' is the wordplay.
'river' becomes 'dee' (River Dee).
'crosses' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'dee' placed around 'n' is 'DENE'.
(Other definitions for dene that I've seen before include "Narrow wooded valley" , "Sandy tract" , "Small valley" , "North American Indian" , "Deep wooded valley" .)