Start being upset (5)
I believe the answer is:
begin
'start' is the definition.
(I know that start can be written as begin)
'being upset' is the wordplay.
'upset' indicates an anagram.
'being' anagrammed gives 'BEGIN'.
(Other definitions for begin that I've seen before include "Israeli statesman" , "Found" , "Commence" , "Get cracking" , "Israeli prime minister and Nobel Peace Prize winner, d. 1992" .)