Old authors with energy for spending (7)
I believe the answer is:
expense
'spending' is the definition.
(I know that spending can be written as expense)
'old authors with energy' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'authors' becomes 'pens' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'ex'+'pens'+'e'='EXPENSE'
'for' acts as a link.
(Other definitions for expense that I've seen before include "Outlay or cost" , "Cost - price" , "Disbursal" , "Disbursement" , "damage" .)