Writer sitting by river to become more profound (6)
I believe the answer is:
deepen
'become more profound' is the definition.
(I've seen this before)
'writer sitting by river' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'sitting by' says to put letters next to each other.
'river' becomes 'dee' (River Dee).
'pen' put after 'dee' is 'DEEPEN'.
'to' acts as a link.
(Other definitions for deepen that I've seen before include "Dig out" , "Increase in depth" , "Intensify" , "Go lower" , "Make stronger" .)