Occasion to have fun, that is, in fast style (6)
I believe the answer is:
fiesta
'occasion' is the definition.
(I know that fiesta is a type of party)
'that is in fast style' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'in' means one lot of letters goes inside another.
'style' indicates an anagram (style can mean to shape or design).
'fast' with letters rearranged gives 'fsta'.
'ie' put into 'fsta' is 'FIESTA'.
'to have fun' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for fiesta that I've seen before include "Religious festival in Hispanic communities" , "religious celebration" , "Outdoor celebration" , "A public holiday in Barcelona" , "(Spanish) religious festival" .)