Arthur's most perfect supporter had left party (4)
I believe the answer is:
gala
'party' is the definition.
(I know that party can be written as gala)
'arthur's most perfect supporter had left' is the wordplay.
'arthur's most perfect supporter' becomes 'Galahad' (Sir Galahad, knight of King Arthur's round table).
'left' is a deletion indicator.
'galahad' with 'had' taken away is 'GALA'.
(Other definitions for gala that I've seen before include "Jamboree" , "Special performance" , "Festive event" , "Sports meeting" , "Festival" .)