Affair began uneasily after quiet (7)
I believe the answer is:
shebang
'affair' is the definition.
('shebang' can informally refer to a matter or situ)
'began uneasily after quiet' is the wordplay.
'uneasily' indicates an anagram.
'after' means one lot of letters go next to another.
'quiet' becomes 'sh'.
'began' anagrammed gives 'ebang'.
'ebang' put after 'sh' is 'SHEBANG'.
(Other definitions for shebang that I've seen before include "Whole affair" , "Entire operation" , "The entire system" , "Matter" , "The whole ___ (entirely)" .)