Vast financial deficit in output from colliery (8)
I believe the answer is:
colossal
'vast' is the definition.
(I know that vast can be written as colossal)
'financial deficit in output from colliery' is the wordplay.
'financial deficit' becomes 'loss' (as in making a loss).
'in' is an insertion indicator.
'output from colliery' becomes 'coal' (a colliery produces coal).
'loss' inserted into 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Stupendous" , "Enormous, gigantic" , "Giant" , "Great" , "Huge, vast" .)