Spend penny — allowed to go in river (7)
I believe the answer is:
deplete
'spend' is the definition.
(depleting is a kind of spending)
'penny allowed to go in river' is the wordplay.
'penny' becomes 'p'.
'allowed' becomes 'let' (I've seen this before).
'to go in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'p'+'let'='plet'
'plet' going inside 'dee' is 'DEPLETE'.
(Other definitions for deplete that I've seen before include "Use up resources of" , "bankrupt" , "Empty, reduce, exhaust" , "Run down (e.g. stocks)" , "Reduce the number of" .)