Huge deficit beginning to accumulate in depression (8)
I believe the answer is:
colossal
'huge' is the definition.
('colossal' can be a synonym of 'huge')
'deficit beginning to accumulate in depression' is the wordplay.
'deficit' becomes 'loss' (I've seen this before).
'beginning to' says to take the initial letters.
'in' is an insertion indicator.
'depression' becomes 'col' (geographical term for a low point).
The first letter of 'accumulate' is 'a'.
'loss'+'a'='lossa'
'lossa' inserted inside 'col' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Stupendous" , "Mammoth" , "Enormous, gigantic" , "Very big" , "Great" .)