Month of subtle charm (5)

I believe the answer is:

march

I'm a little stuck... Click here to teach me more about this clue!

'month' is the definition.
(I know that March is a type of Gregorian calendar month)

'subtle charm' is the wordplay.
'charm' anagrams to 'MARCH'.
However, I'm not sure how this is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for march that I've seen before include "Moving demonstration" , "demo" , "Month, piece of music" , "Month - proceed" , "Journey undertaken by army etc" .)

I've seen this clue in the Evening Standard.

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