Jack cheers Romeo (3)
I believe the answer is:
tar
'jack' is the definition.
(thesaurus)
'cheers romeo' is the wordplay.
'cheers' becomes 'ta' (I've seen this in other clues).
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'ta'+'r'='TAR'
(Other definitions for tar that I've seen before include "Naval man" , "Crewman" , "Thick flammable liquid" , "Mariner, old salt" , "Cigarette substance" .)