Start being messed about (5)
I believe the answer is:
begin
'start' is the definition.
(I know that start can be written as begin)
'being messed about' is the wordplay.
'messed about' indicates an anagram.
'being' is an anagram of 'BEGIN'.
(Other definitions for begin that I've seen before include "Monachem --" , "Found" , "Israeli statesman" , "former Israeli Nobel peace prize winner" , "Get cracking" .)