Firm reduced by half since company's disaster (6)
I believe the answer is:
fiasco
'disaster' is the definition.
('fiasco' can be a synonym of 'disaster')
'firm reduced by half since company's' is the wordplay.
'reduced by half' means to take half the letters.
'since' becomes 'as' (I've seen this in other clues).
'company' becomes 'co' (abbreviation).
'firm' with half the letters taken is 'fi'.
'fi'+'as'+'co'='FIASCO'
(Other definitions for fiasco that I've seen before include "Ridiculous and total failure" , "Ignominious failure" , "Total cock-up" , "Ignominious result" , "An absolute mess, what a farce" .)