Wakes up when crossing River Ouse (7)
I believe the answer is:
arouses
'wakes up' is the definition.
('arouse' can be a synonym of 'wake up')
'when crossing river ouse' is the wordplay.
'when' becomes 'as'.
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'r'+'ouse'='rouse'
'as' placed around 'rouse' is 'AROUSES'.
(Other definitions for arouses that I've seen before include "gets excited" , "Inspires" , "Stimulates" .)