The cost of keeping swans in the river (7)
I believe the answer is:
expense
'the cost' is the definition.
(the cost or price of something)
'keeping swans in the river' is the wordplay.
'keeping' means one lot of letters goes inside another.
'swans in' becomes 'pens' (pen is a kind of swan. I am not sure about the 'in' bit.).
'the river' becomes 'exe' (River Exe).
'pens' placed inside 'exe' is 'EXPENSE'.
'of' is the link.
Can you help me to learn more?
(Other definitions for expense that I've seen before include "Incurred cost" , "Disbursal" , "Charge" , "Money spent in the course of work" , "damage" .)