Grand, in the style of festive event (4)
I believe the answer is:
gala
'festive event' is the definition.
(I've seen this before)
'grand in the style of' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'in the style of' becomes 'ala' (I've seen this before).
'g'+'ala'='GALA'
(Other definitions for gala that I've seen before include "Festival" , "Public festivity" , "Festive event" , "Festive performance" , "A show of charity" .)