Hideaway by river in London (6)
I believe the answer is:
camden
'in london' is the definition.
(area of London)
'hideaway by river' is the wordplay.
'hideaway' becomes 'den' (synonyms).
'by' means one lot of letters go next to another.
'river' becomes 'cam' (river in Cambridge, England).
'den' put after 'cam' is 'CAMDEN'.
(Other definitions for camden that I've seen before include "Inner London borough" , "Dublin street and London borough" , "London borough, includes St Pancras" , "Area of north London" .)