Want master dumped in river? (6)
I believe the answer is:
desire
'want' is the definition.
(I know that want can be written as desire)
'master dumped in river?' is the wordplay.
'master' becomes 'sir' (a schoolmaster may be addressed as sir).
'dumped in' means one lot of letters goes inside another.
'river?' becomes 'dee' (River Dee).
'sir' inserted inside 'dee' is 'DESIRE'.
(Other definitions for desire that I've seen before include "Want to reside?" , "Wish for strongly" , "Long for (something)" , "Longing or wish" , "Covet" .)