Cost where pigs are kept in river (7)
I believe the answer is:
expense
'cost' is the definition.
(the cost or price of something)
'pigs are kept in river' is the wordplay.
'pigs are' becomes 'pens' (I can't justify this - if you can you should believe this answer much more).
'kept in' is an insertion indicator.
'river' becomes 'exe' (River Exe).
'pens' put inside 'exe' is 'EXPENSE'.
'where' acts as a link.
Can you help me to learn more?
(Other definitions for expense that I've seen before include "Disbursal" , "Disbursement" , "Charge" , "Money spent in the course of work" , "damage" .)