Sailor on board performs exceptionally (5)
I believe the answer is:
stars
'performs exceptionally' is the definition.
'stars' can be an answer for 'performs' (starring is a kind of performing). I'm unsure of the 'exceptionally' bit.
'sailor on board' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'on' is an insertion indicator.
'board' becomes 'ss' (I've seen this in other clues).
'tar' placed within 'ss' is 'STARS'.
Can you help me to learn more?
(Other definitions for stars that I've seen before include "Horoscope (colloq.)" , "Celebs - constellation" , "Leading actors" , "Heavenly bodies" , "Celestial bodies of hot gases" .)