Fundamental course is underground (7,4)

I believe the answer is:

central line

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'underground' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'fundamental course' is the wordplay.
'fundamental' becomes 'central' (synonyms).
'course' becomes 'line' (synonyms).
'central'+'line'='CENTRAL LINE'

'is' acts as a link.

Can you help me to learn more?

(Another definition for central line that I've seen is " West Ruislip to Epping Tube route".)

I've seen this clue in The Mirror.

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