River, extremely rough one (5)
I believe the answer is:
rhone
'river' is the definition.
(Rhone River is an example)
'extremely rough one' is the wordplay.
'extremely' suggests removing the centre.
'rough' with its centre removed is 'rh'.
'rh'+'one'='RHONE'
(Other definitions for rhone that I've seen before include "River of Switzerland and France" , "River flowing through Lake Geneva" , "wherein runs rain in Arbroath" , "River running through the city of Arles" , "Major French river, rising in Switzerland" .)