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' means one lot of letters go next to another.
'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" .)