Dirty little devil by a river (6)
I believe the answer is:
impure
'dirty' is the definition.
(I've seen this before)
'little devil by a river' is the wordplay.
'little devil' becomes 'imp' (an imp is a little demon).
'by' says to put letters next to each other.
'a river' becomes 'ure' (river in Yorkshire).
'imp'+'ure'='IMPURE'
(Other definitions for impure that I've seen before include "Assign a value to" , "Contaminated or licentious" , "Adulterated, dirty (6)" , "Sullied or adulterated" , "Lewd" .)