Grand tableau regularly used for celebration (4)
I believe the answer is:
gala
'for celebration' is the definition.
(gala is a kind of celebration)
'grand tableau regularly' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'tableau' are 'ala'.
'g'+'ala'='GALA'
'used' acts as a link.
(Other definitions for gala that I've seen before include "Public festivity" , "Festival" , "Carnival, fete" , "Festive entertainment" , "Festivity - celebration" .)