Southern mariner's guiding light (4)
I believe the answer is:
star
'guiding light' is the definition.
(stars can be used for navigation)
'southern mariner's' is the wordplay.
'southern' becomes 'S' (abbreviation).
'mariner' becomes 'tar' (synonyms).
's'+'tar'='STAR'
(Other definitions for star that I've seen before include "Celebrity in the world of entertainment" , "Heavenly apparition" , "chamber" , "Heavenly body - leading actress?" , "Celebrity - shining light" .)