Antagonise cult, capturing a very strong princess first (9)
I believe the answer is:
disaffect
'antagonise' is the definition.
'cult capturing a very strong princess first' is the wordplay.
'cult' becomes 'sect' (I've seen this before).
'capturing' indicates putting letters inside.
'very strong' becomes 'ff' (musical abbreviation for fortissimo, 'very strong').
'princess' becomes 'di'.
'first' is a charade indicator (letters next to each other).
'a'+'ff'='aff'
'sect' placed around 'aff' is 'saffect'.
'saffect' after 'di' is 'DISAFFECT'.
(Another definition for disaffect that I've seen is " Alienate".)