Grand, in the manner of festival (4)
I believe the answer is:
gala
'festival' is the definition.
(a gala is a festival or party)
'grand in the manner of' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'in the manner of' becomes 'a la' (e.g. chicken a la King).
'g'+'ala'='GALA'
(Other definitions for gala that I've seen before include "sporting event" , "Special social or sporting occasion" , "A festive occasion, perhaps of sporting nature" , "Special sports event" , "Carnival, fete" .)