Master the right amount of grammar! (3)

I believe the answer is:

mar

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

'amount of grammar' is the definition.
'Mar' can be an answer for 'amount' (Mar is an example). I'm unsure of the 'of grammar' bit.

'master the right' is the wordplay.
'master' becomes 'ma' (Master of Arts).
'the right' becomes 'r' (common abbreviation).
'ma'+'r'='MAR'

Can you help me to learn more?

(Other definitions for mar that I've seen before include "when spring growth starts?" , "Spoil something" , "hurt" , "wreck" , "'Impair, spoil (3)'" .)

I've seen this clue in the Evening Standard.

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