Work persistently beside river with great intensity (6)
I believe the answer is:
deeply
'with great intensity' is the definition.
(I've seen this in another clue)
'work persistently beside river' is the wordplay.
'work persistently' becomes 'ply' (as in plying a trade).
'beside' says to put letters next to each other (I've seen this in other clues).
'river' becomes 'dee' (River Dee).
'ply' put after 'dee' is 'DEEPLY'.
(Other definitions for deeply that I've seen before include "Intensely" , "Thoroughly" , "With profundity" , "With intense feeling" , "Profoundly" .)