Thanks to Romeo, he’s called Jack (3)
I believe the answer is:
tar
'he's called jack' is the definition.
(jack and tar are terms for a sailor)
'thanks to romeo' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'ta'+'r'='TAR'
(Other definitions for tar that I've seen before include "Cigarette residue - old word for sailor" , "A sailor; a preservative" , "Sailor; road surface" , "Smoker's black deposit" , "Familiar sailor - what a rat" .)