Grand, in the manner of festivity (4)
I believe the answer is:
gala
'festivity' is the definition.
(gala is a kind of festivity)
'grand in the manner of' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'in the manner of' becomes 'ala' (I've seen this before).
'g'+'ala'='GALA'
(Other definitions for gala that I've seen before include "Carnival, fete" , "Festive event" , "Swimming competition" , "Festive performance" , "Festive occasion - variety of apple" .)