Cost of keeping female 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 not certain of the 'of' bit.
'female swans in river?' is the wordplay.
'female swans' becomes 'pens' (a female swan is a pen).
'in' is an insertion indicator.
'river?' becomes 'exe' (River Exe).
'pens' put within 'exe' is 'EXPENSE'.
'keeping' acts as a link.
(Other definitions for expense that I've seen before include "Charge" , "Disbursement" , "Cost - price" , "Incurred cost" , "Disbursal" .)