One guiding Southern sailor (4)
I believe the answer is:
star
'one guiding' is the definition.
(I've seen this before)
'southern sailor' is the wordplay.
'southern' becomes 's' (abbreviation).
'sailor' becomes 'tar' (tar can mean a sailor).
's'+'tar'='STAR'
(Other definitions for star that I've seen before include "See 21" , "Main attraction" , "Eg, Rigel, Aldebaran" , "Outstanding entertainer" , "feature in film" .)