Rousing song that is a standard (3,4)

I believe the answer is:

red flag

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'rousing song' is the definition.
The definition suggests a singular noun which matches the answer.

'is a standard' is the wordplay.
'is a' becomes 'red' (I am not sure about this - if you are sure you should believe this answer much more).
'standard' becomes 'flag' (I've seen this in another clue).
'red'+'flag'='RED FLAG'

'that' is the link.

Can you help me to learn more?

(Other definitions for red flag that I've seen before include "Socialist banner?" , "Sign of danger - sign of socialism" , "Warning sign" , "Socialist symbol" , "Emblem of socialist revolution or danger" .)

I've seen this clue in The Globe and Mail.

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