Star's promise in Union and League (4)
I believe the answer is:
idol
'star's' is the definition.
(idol is a kind of star)
'promise in union and league' is the wordplay.
'promise in union' becomes 'ido' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'league' becomes 'l' (abbreviation).
'ido'+'l'='IDOL'
Can you help me to learn more?
(Other definitions for idol that I've seen before include "As before, fake" , "Heathen god" , "Effigy" , "Graven image or greatly admired person" , "Someone much loved" .)