Highlands canal firm's set for engineering (6,7)

I believe the answer is:

massif central

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'highlands' is the definition.
(I know that Massif Central is a plateau)

'canal firm's set for engineering' is the wordplay.
'canal firm's set' anagrams to 'MASSIF CENTRAL'.
Though, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for massif central that I've seen before include "Mountainous plateau covering a sixth of France" , "Mountainous plateau of S. central France" , "Mountainous plateau of southern France" , "Upland area in southern France" .)

I've seen this clue in The Independent.

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