Outlay keeping female swans in river (7)
I believe the answer is:
expense
'outlay' is the definition.
(synonyms)
'female swans in river' is the wordplay.
'female swans' becomes 'pens' (I've seen this before).
'in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'pens' placed into 'exe' is 'EXPENSE'.
'keeping' is the link.
(Other definitions for expense that I've seen before include "Incurred cost" , "Outlay or cost" , "damage" , "Disbursement" , "What something costs" .)