The extent of a mega-section (5)

I believe the answer is:

ambit

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'the extent of' is the definition.
(I've seen this before)

'a mega-section' is the wordplay.
'mega' becomes 'm' (this might be a standard abbreviation I've not previously seen).
'section' becomes 'bit' (both can mean part).
'a'+'m'+'bit'='AMBIT'

Can you help me to learn more?

(Other definitions for ambit that I've seen before include "Compass, scope" , "Scope, extent, or bounds of something" , "Circumference" , "Circuit, compass" , "round" .)

I've seen this clue in the Sydney Morning Herald.

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