A river with female swans becomes less shallow (7)
I believe the answer is:
deepens
'becomes less shallow' is the definition.
(I've seen this in another clue)
'a river with female swans' is the wordplay.
'a river' becomes 'dee' (River Dee).
'with' is a charade indicator (letters next to each other).
'female swans' becomes 'pens' (a female swan is a pen).
'dee'+'pens'='DEEPENS'
(Other definitions for deepens that I've seen before include "Dredges" , "gets stronger" , "Make further down, say ditch" , "Intensifies" , "increases" .)