Masterly mix-up, this motto? (5)

I believe the answer is:

maxim

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'motto?' is the definition.
(thesaurus)

'masterly mix-up' is the wordplay.
'masterly' becomes 'ma' (I am not sure about this - if you are sure you should believe this answer much more).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'mix' reversed gives 'xim'.
'ma'+'xim'='MAXIM'

'this' acts as a link.

Can you help me to learn more?

(Other definitions for maxim that I've seen before include "Short pithy saying" , "Proverb, aphorism" , "Old machine-gun" , "Saying, adage" , "Saying - inventor of machine gun" .)

I've seen this clue in the Evening Standard.

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