Festivity with experienced knight (7)
I believe the answer is:
galahad
'knight' is the definition.
(I've seen this before)
'festivity with experienced' is the wordplay.
'festivity' becomes 'gala' (gala is a kind of festivity).
'with' says to put letters next to each other.
'experienced' becomes 'had' (having is a kind of experiencing).
'gala'+'had'='GALAHAD'
(Other definitions for galahad that I've seen before include "the noblest of men" , "extremely honourable man" , "Round Table knight destined to retrieve the Holy Grail" , "Errant fellow" , "The most virtuous knight at the Arthurian Round Table" .)