River gets writer to grow (6)
I believe the answer is:
deepen
'to grow' is the definition.
(I've seen this before)
'river gets writer' is the wordplay.
'river' becomes 'dee' (River Dee).
'gets' says to put letters next to each other.
'writer' becomes 'pen' (something which writes).
'dee'+'pen'='DEEPEN'
(Other definitions for deepen that I've seen before include "Intensify" , "make room for more water" , "To grow" , "Get stronger" , "Increase in depth" .)