I refuse to lighten up the linear magnitude (6)
I believe the answer is:
length
'magnitude' is the definition.
(length is a kind of magnitude)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for length that I've seen before include "Extent from end to end" , "Measurement from end to end" , "Linear extent" , "stretch" , "End-to-end distance" .)