Firm reduced by half since company's complete failure (6)
I believe the answer is:
fiasco
'complete failure' is the definition.
(I've seen this before)
'firm reduced by half since company's' is the wordplay.
'reduced by half' suggests halving the letters.
'since' becomes 'as' (I've seen this in other clues).
'company' becomes 'co' (abbreviation).
'firm' cut in half is 'fi'.
'fi'+'as'+'co'='FIASCO'
(Other definitions for fiasco that I've seen before include "Ignominious result" , "Complete disaster" , "bomb" , "Ignominious failure" , "Sudden total disaster" .)