Extremely rash individual in river (5)
I believe the answer is:
rhone
'river' is the definition.
(Rhone River is an example)
'extremely rash individual' is the wordplay.
'extremely' suggests removing the centre.
'individual' becomes 'one'.
'rash' with its middle removed is 'rh'.
'rh'+'one'='RHONE'
'in' is the link.
(Other definitions for rhone that I've seen before include "River at Avignon and Arles" , "River running from the Swiss Alps to the Mediterranean" , "French flower" , "wherein runs rain in Arbroath" , "River of Switzerland and France" .)