Sicilian shepherd about to go north on island (4)
I believe the answer is:
acis
'sicilian shepherd' is the definition.
(mythical figure)
'about to go north on island' is the wordplay.
'about' becomes 'ca' (short for circa).
'to go north' says the letters should be written in reverse (I've seen 'going north' mean this).
'on' is a charade indicator (letters next to each other).
'island' becomes 'is' (abbreviation).
'ca' written backwards gives 'ac'.
'ac'+'is'='ACIS'
(Other definitions for acis that I've seen before include "--- and Galatea (Handel)" , "Sicilian lover" , "shepherd" , "giant" , "Victim of Cyclops" .)