Smooth outlaw in Yorkshire river (6)
I believe the answer is:
urbane
'smooth' is the definition.
(I've seen this before)
'outlaw in yorkshire river' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'in' indicates putting letters inside.
'yorkshire river' becomes 'ure' (River Ure).
'ban' going into 'ure' is 'URBANE'.
(Other definitions for urbane that I've seen before include "Polished -- sophisticated" , "Cosmopolitan" , "Refined and civilised" , "Elegant and sophisticated - sounds like city type" , "Suave and sophisticated" .)