Sailor in southern city having rest day (7)
I believe the answer is:
sabbath
'rest day' is the definition.
(Sabbath is an example)
'sailor in southern city' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'city' becomes 'bath' (English city).
's'+'bath'='sbath'
'ab' placed into 'sbath' is 'SABBATH'.
'having' acts as a link.
(Other definitions for sabbath that I've seen before include "Sacred day of the week" , "Traditional holy day" , "Religious day of rest for Jews and Christians" , "Hebrew word for day of rest" , "Jewish rest-day" .)