Message half of us get so wrong! (3)
I believe the answer is:
sos
'message' is the definition.
'half of us get so wrong' is the wordplay.
'half of' means to take half the letters.
'get' is a charade indicator (letters next to each other).
'wrong' indicates anagramming the letters.
'us' cut in half is 's'.
'so' is an anagram of 'os'.
's'+'os'='SOS'
(Other definitions for sos that I've seen before include "Mayday call" , "Call for help" , "Appeal for help" , "Coded distress signal" , "Help code in 13 Across" .)