Jack's a bit of a star (3)
I believe the answer is:
tar
'jack's' is the definition.
(both are informal terms for sailor)
'a bit of a star' is the wordplay.
'TAR' is hidden within the letters.
However, I'm not sure how this is indicated.
Can you help me to learn more?
(Other definitions for tar that I've seen before include "Sticky cigarette residue" , "Dark flammable liquid" , "Thick, sticky liquid distilled from coal" , "Cigarette residue - old word for sailor" , "Old salt or liquid from coal" .)