Cost of putting swans in river (7)
I believe the answer is:
expense
'cost of' is the definition.
'expense' can be an answer for 'cost' (the cost or price of something). I'm unsure of the 'of' bit.
'swans in river' is the wordplay.
'swans' becomes 'pens' (pen can mean a female swan).
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'pens' put within 'exe' is 'EXPENSE'.
'putting' is the link.
Can you help me to learn more?
(Other definitions for expense that I've seen before include "Disbursement" , "Disbursal" , "damage" , "Outlay or cost" , "Money spent in the course of work" .)