Start being silly (5)
I believe the answer is:
begin
'start' is the definition.
('begin' can be a synonym of 'start')
'being silly' is the wordplay.
'silly' indicates an anagram.
'being' with letters rearranged gives 'BEGIN'.
(Other definitions for begin that I've seen before include "Make a start" , "Institute" , "Israeli statesman" , "former Israeli Nobel peace prize winner" , "Get cracking" .)