Work behind river in a profound way (6)
I believe the answer is:
deeply
'in a profound way' is the definition.
(I've seen this before)
'work behind river' is the wordplay.
'work' becomes 'ply' (I've seen this before).
'behind' means one lot of letters go next to another.
'river' becomes 'Dee' (River Dee).
'ply' after 'dee' is 'DEEPLY'.
(Other definitions for deeply that I've seen before include "In a profound way" , "Profoundly" , "Intensely" , "With profundity" , "With intense feeling" .)