Boatman goes on south to get sun? (4)
I believe the answer is:
star
'sun?' is the definition.
(sun is a kind of star)
'boatman goes on south' is the wordplay.
'boatman' becomes 'tar' (tar is an informal term for a sailor).
'goes on' means one lot of letters go next to another.
'south' becomes 's' (synonyms).
'tar' after 's' is 'STAR'.
'to get' acts as a link.
(Other definitions for star that I've seen before include "Famous performer" , "Top celebrity" , "Sirius, eg" , "Celebrity - heavenly body" , "Famous actor" .)