Onset of blocked sinuses ruined affair (8)
I believe the answer is:
business
'affair' is the definition.
(thesaurus)
'onset of blocked sinuses ruined' is the wordplay.
'onset of' suggests taking the first letters.
'ruined' is an anagram indicator.
The initial letter of 'blocked' is 'b'.
'sinuses' anagrammed gives 'usiness'.
'b'+'usiness'='BUSINESS'
(Other definitions for business that I've seen before include "Baby" , "Affair - matters to be attended to" , "Occupation; trade" , "See 1" , "Commerce, trade" .)