Cost of swans being introduced to river (7)
I believe the answer is:
expense
'cost' is the definition.
(thesaurus)
'swans being introduced to river' is the wordplay.
'swans' becomes 'pens' (pen is a kind of swan).
'being introduced to' indicates putting letters inside.
'river' becomes 'exe' (I've seen this before).
'pens' placed into 'exe' is 'EXPENSE'.
'of' acts as a link.
(Other definitions for expense that I've seen before include "Disbursal" , "Outlay or cost" , "Charge" , "Disbursement" , "Incurred cost" .)