Brilliant Bill leaves laughing (5)
I believe the answer is:
riant
'laughing' is the definition.
('riant' can be a synonym of 'laughing')
'brilliant bill leaves' is the wordplay.
'leaves' suggests deleting specific letters.
'brilliant' with 'bill' taken away is 'RIANT'.
(Other definitions for riant that I've seen before include "Laughing" , "merry" , "patently amused" .)