Cost of swans kept in river (7)
I believe the answer is:
expense
'cost' is the definition.
(thesaurus)
'swans kept in river' is the wordplay.
'swans' becomes 'pens' (pen is a kind of swan).
'kept in' means one lot of letters goes inside another.
'river' becomes 'exe' (I've seen this before).
'pens' placed within 'exe' is 'EXPENSE'.
'of' is the link.
(Other definitions for expense that I've seen before include "Charge" , "Disbursement" , "Disbursal" , "damage" , "Outlay or cost" .)