Dirty little devil on the river (6)
I believe the answer is:
impure
'dirty' is the definition.
(thesaurus)
'little devil on the river' is the wordplay.
'little devil' becomes 'imp' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'the river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'imp'+'ure'='IMPURE'
(Other definitions for impure that I've seen before include "Unclean" , "Sullied or adulterated" , "Adulterated, dirty (6)" , "Defiled" , "Adulterated, tainted" .)