Performance - former jester's last in court (7)
I believe the answer is:
concert
'performance' is the definition.
(I have seen 'Musical performance ' mean 'concert' so perhaps 'performance' could also mean 'concert')
'former jester's last in court' is the wordplay.
'former' becomes 'once' ('once' can be a synonym of 'former'**).
'last' says to take the final letters.
'in' is an insertion indicator.
'court' becomes 'ct'.
The final letter of 'jester' is 'r'.
'once'+'r'='oncer'
'oncer' going within 'ct' is 'CONCERT'.
(Other definitions for concert that I've seen before include "Harmony, performance" , "Public performance of music" , "Musical event" , "Joint action" , "Harmony of people's opinions, actions or characters" .)