Jack co-starring in part (3)
I believe the answer is:
tar
'jack' is the definition.
(thesaurus)
'co-starring in part' is the wordplay.
'in part' says the answer is hidden in the clue.
'TAR' can be found hidden inside 'co starring'.
(Other definitions for tar that I've seen before include "used by those who pay to work on deck" , "Sticky black stuff" , "Flammable liquid; sailor" , "Kind of rat for sailor" , "one may need to pay on deck" .)