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' is a charade indicator (letters next to each other).
'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" .)