Brilliance in act was highly entertaining (9)
I believe the answer is:
delighted
'was highly entertaining' is the definition.
(delight can mean to entertain greatly)
'brilliance in act' is the wordplay.
'brilliance' becomes 'light' (brilliant can mean shining or lit up).
'in' indicates putting letters inside.
'act' becomes 'deed' (deed is a kind of act).
'light' put into 'deed' is 'DELIGHTED'.
(Other definitions for delighted that I've seen before include "Greatly pleased" , "Pleased as Punch" , "Joyful" , "Overjoyed" , "Over the moon" .)