Head of Security's husband began faltering affair (7)
I believe the answer is:
shebang
'affair' is the definition.
('shebang' can informally refer to a matter or situ)
'head of security's husband began faltering' is the wordplay.
'head of' indicates taking the first letters.
'faltering' indicates an anagram.
The initial letters of 'securitys husband' is 'sh'.
'began' with letters rearranged gives 'ebang'.
'sh'+'ebang'='SHEBANG'
(Other definitions for shebang that I've seen before include "Entire operation" , "Lot" , "(Whole) kit and caboodle" , "business" , "Whole affair" .)