Gang member in musical based on Romeo (7)
I believe the answer is:
greaser
'gang member' is the definition.
(member of a motorcycle gang)
'musical based on romeo' is the wordplay.
'musical' becomes 'grease' (Grease is a film musical).
'based on' means one lot of letters go next to another.
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'grease'+'r'='GREASER'
'in' is the link.
(Other definitions for greaser that I've seen before include "Young motorcyclist" , "(Unskilled) mechanic" , "An unctuous individual" , "Mechanic - sycophant" , "Ship's engineer; motor mechanic" .)