Fellow is concealing a company - it's a disaster (6)
I believe the answer is:
fiasco
'it's a disaster' is the definition.
('fiasco' can be a synonym of 'disaster')
'fellow is concealing a company' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'concealing' is an insertion indicator.
'company' becomes 'co' (abbreviation).
'is' placed around 'a' is 'ias'.
'f'+'ias'+'co'='FIASCO'
(Other definitions for fiasco that I've seen before include "Debacle, a right mess" , "A ludicrous or humiliating failure, complete mess" , "Humiliating disaster" , "A hopeless mess or ludicrous failure" , "Ridiculous and total failure" .)