Calm those upset about nothing (6)
I believe the answer is:
soothe
'calm' is the definition.
(thesaurus)
'those upset about nothing' is the wordplay.
'upset' indicates anagramming the letters.
'about' indicates putting letters inside.
'nothing' becomes 'o' (looks like zero - 0).
'those' is an anagram of 'sothe'.
'sothe' going around 'o' is 'SOOTHE'.
(Other definitions for soothe that I've seen before include "Gently calm" , "to settle" , "Calm down, placate" , "Cause to feel better" , "Calm; relieve pain" .)