Shrug off Sid coming back with unmarried woman (7)
I believe the answer is:
dismiss
'shrug off' is the definition.
(thesaurus)
'sid coming back with unmarried woman' is the wordplay.
'coming back' shows that the letters should be reversed in order.
'with' means one lot of letters go next to another.
'unmarried woman' becomes 'miss' (as opposed to Mrs).
'sid' reversed gives 'dis'.
'dis'+'miss'='DISMISS'
(Other definitions for dismiss that I've seen before include "Throw out" , "P45" , "military order" , "Drop" , "Remove from office" .)