Guard what one says - it's a good maxim (9)

I believe the answer is:

watchword

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'one says it's a good maxim' is the definition.
I can't judge whether this definition defines the answer.

'guard what' is the wordplay.
'guard' becomes 'watch' (watch is a kind of guard).
'what' becomes 'word' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'watch'+'word'='WATCHWORD'

Can you help me to learn more?

(Other definitions for watchword that I've seen before include "rallying-cry" , "Slogan" .)

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

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