Imaginary line originally ran halfway under Mont Blanc (5)

I believe the answer is:

rhumb

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'imaginary line' is the definition.
'rhumb' can be an answer for 'line' (rhumb is a kind of line). I'm not certain of the 'imaginary' bit.

'originally ran halfway under mont blanc' is the wordplay.
'originally' says to take the initial letters.
The initial letters of 'ran halfway under mont blanc' is 'RHUMB'.

Can you help me to learn more?

(Other definitions for rhumb that I've seen before include "Course following a uniform compass heading" , "Any point of the compass - fixed line on a chart" .)

I've seen this clue in The Independent.

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