Single day after middle of June (5)
I believe the answer is:
unwed
'single' is the definition.
('unwed' can be a synonym of 'single')
'day after middle of june' is the wordplay.
'day' becomes 'wed' (abbreviation for Wednesday).
'after' means one lot of letters go next to another.
'middle of' means to look at the middle letters.
The central letters of 'june' are 'un'.
'wed' put after 'un' is 'UNWED'.
(Other definitions for unwed that I've seen before include "Single" , "Divorced" , "Not married" , "Without benefit of clergy?" .)