Leading sailor following sun (4)
I believe the answer is:
star
'leading' is the definition.
('star' can be a synonym of 'leading')
'sailor following sun' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'following' means one lot of letters go next to another.
'sun' becomes 's'.
'tar' after 's' is 'STAR'.
(Other definitions for star that I've seen before include "Celebrated actor - heavenly body!" , "appear above others?" , "It twinkles" , "See 21" , "Polaris, for example" .)