Former writer’s energy outlay (7)
I believe the answer is:
expense
'outlay' is the definition.
('expense' can be a synonym of 'outlay')
'former writer's energy' is the wordplay.
'former' becomes 'ex'.
'writer's' becomes 'pens'.
'energy' becomes 'e' (physics symbol as in E=mc2).
'ex'+'pens'+'e'='EXPENSE'
(Other definitions for expense that I've seen before include "damage" , "Incurred cost" , "Disbursement" , "Outlay or cost" , "Money spent in the course of work" .)