Carol and Leslie - they aren't married (7)
I believe the answer is:
singles
'they aren't married' is the definition.
'carol and leslie' is the wordplay.
'carol' becomes 'sing' (carol can mean to sing carols).
'and' means one lot of letters go next to another.
'leslie' becomes 'les' (short for Leslie).
'sing'+'les'='SINGLES'
(Other definitions for singles that I've seen before include "Form of tennis" , "Type of tennis match" , "Non-return tickets" , "Ones" .)