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' means one lot of letters goes inside another.
'yorkshire river' becomes 'ure' (River Ure).
'ban' going within '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" .)