Former writer’s ultimately excessive outlay (7)
I believe the answer is:
expense
'outlay' is the definition.
('expense' can be a synonym of 'outlay')
'former writer's ultimately excessive' is the wordplay.
'former' becomes 'ex'.
'writer's' becomes 'pens' (**).
'ultimately' suggests the final letters.
The last letter of 'excessive' is 'e'.
'ex'+'pens'+'e'='EXPENSE'
(Other definitions for expense that I've seen before include "What something costs" , "Disbursal" , "Cost - price" , "damage" , "Incurred cost" .)