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' suggests halving the letters.
'get' means one lot of letters go next to another.
'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" .)