Writers in river demanding fee (7)
I believe the answer is:
expense
'fee' is the definition.
(thesaurus)
'writers in river' is the wordplay.
'writers' becomes 'pens' (I've seen this before).
'in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'pens' put into 'exe' is 'EXPENSE'.
'demanding' is the link.
(Other definitions for expense that I've seen before include "Money spent in the course of work" , "Charge" , "Outlay or cost" , "Cost - price" , "What something costs" .)