Cancel changes in South Vietnamese celebration (4)
I believe the answer is:
stet
'cancel changes' is the definition.
The definition suggests a singular noun which matches the answer.
'south vietnamese celebration' is the wordplay.
'south' becomes 's' ('S' can be a synonym of 'south').
'vietnamese celebration' becomes 'tet' (I can't justify this - if you can you should give a lot more credence to this answer).
's'+'tet'='STET'
'in' is the link.
Can you help me to learn more?
(Other definitions for stet that I've seen before include "This must not be changed" , "Instruction to cancel a deletion" , "Print as is, don't alter spelling" , "As written, don't change" , "Proofreader's mark that the original text should be kept" .)