Defeat flotilla occasionally (4)
I believe the answer is:
foil
'defeat' is the definition.
(thesaurus)
'flotilla occasionally' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternating letters of 'flotilla' are 'FOIL'.
(Other definitions for foil that I've seen before include "Prevent from succeeding" , "Baffle" , "Thin metal - sword" , "Hinder - prevent" , "Piece of thin and flexible sheet metal" .)