Legendary knight had forgotten festive occasion (4)
I believe the answer is:
gala
'festive occasion' is the definition.
(I've seen this before)
'legendary knight had forgotten' is the wordplay.
'legendary knight' becomes 'galahad' (Sir Galahad, knight of the Round Table).
'forgotten' suggests deleting specific letters.
'galahad' with 'had' removed is 'GALA'.
(Other definitions for gala that I've seen before include "Festive event" , "Festive performance" , "Fête" , "A festive occasion, perhaps of sporting nature" , "Jamboree" .)