Wishes knight in river success at first (7)
I believe the answer is:
desires
'wishes' is the definition.
(desiring is a kind of wishing)
'knight in river success at first' is the wordplay.
'knight' becomes 'sir' (a knight has the title Sir).
'in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'at first' indicates taking the first letters.
The first letter of 'success' is 's'.
'sir' placed within 'dee' is 'desire'.
'desire'+'s'='DESIRES'
(Other definitions for desires that I've seen before include "Wishes" , "needs" , "Wants (7)." , "Hopes" , "Covets and yearns for" .)