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