Gang member John Travolta plays as Romeo, after cast agrees (7)
I believe the answer is:
greaser
'gang member' is the definition.
(member of a motorcycle gang)
'romeo after cast agrees' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'after' is a charade indicator (letters next to each other).
'cast' indicates anagramming the letters (letters are dispersed or cast around).
'agrees' is an anagram of 'grease'.
'r' put after 'grease' is 'GREASER'.
'john travolta plays as' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for greaser that I've seen before include "Mechanic - sycophant" , "Ship's engineer; motor mechanic" , "(Unskilled) mechanic" , "Young motorcyclist" , "Biker" .)