Calm down those upset about nothing (6)
I believe the answer is:
soothe
'calm down' is the definition.
(I've seen this before)
'those upset about nothing' is the wordplay.
'upset' indicates an anagram.
'about' indicates putting letters inside.
'nothing' becomes 'o' (O resembles 0).
'those' with letters rearranged gives 'sothe'.
'sothe' placed around 'o' is 'SOOTHE'.
(Other definitions for soothe that I've seen before include "'Console, comfort (6)'" , "Cause to feel better" , "to settle" , "Gently calm" , "Console, pacify" .)