Gang in West Side Story film, travelling socialites? (3,3,3)
I believe the answer is:
the jet set
'travelling socialites?' is the definition.
(rich people who travel by plane a lot)
'gang in west side story film' is the wordplay.
'gang' becomes 'jets' (gang in the musical West Side Story).
'in' is an insertion indicator.
'west side story' becomes 'the' (I am not sure about this - if you are sure you should believe this answer much more).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'the'+'et'='theet'
'jets' going into 'theet' is 'THE JET SET'.
(Other definitions for the jet set that I've seen before include "frequently found on planes?" , "Black box" , "on a 747?" , "Wealthy globetrotters" .)