Gala pie, ultimately, after endless cheese (4)
I believe the answer is:
fete
'gala' is the definition.
(both can mean a festive event)
'pie ultimately after endless cheese' is the wordplay.
'ultimately' indicates one should take the final letters.
'after' means one lot of letters go next to another.
'endless' means to remove the last letter.
'cheese' becomes 'feta' (type of cheese).
The last letter of 'pie' is 'e'.
'feta' with its last letter taken off is 'fet'.
'e' put after 'fet' is 'FETE'.
(Other definitions for fete that I've seen before include "Garden event" , "Outdoor entertainment and sale, often for charity" , "Marilyn Monroe's real first name' constellation" , "Charity garden party" , "Outdoor get-together" .)