Sailor boarding ship? Great balls of fire! (5)
I believe the answer is:
stars
'great balls of fire' is the definition.
(I've seen this in another clue)
'sailor boarding ship?' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'boarding' is an insertion indicator.
'ship?' becomes 'SS' (prefix in ship names eg SS Great Britain).
'tar' put within 'ss' is 'STARS'.
(Other definitions for stars that I've seen before include "Leading performers (with stripes?)" , "plays a leading role" , "Horoscope (colloq.)" , "Heavenly bodies" , "''What is the ...? What is the ...? '' (Juno and the Paycock)" .)