Saboteur losing heart, becoming more high-minded (6)

I believe the answer is:

nobler

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'more high-minded' is the definition.
('noble' can be similar in meaning to 'high-minded')

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for nobler that I've seen before include "More honourable" , "'Whether 'tis ... in the mind, to suffer the slings and arrows of outrageous fortune''/Hamlet" , "More imposing or magnificent" , "More aristocratic" , "''Whether 'tis ... in the mind to suffer / The slings and arrows of outrageous fortune'' (Hamlet)" .)

I've seen this clue in The Independent.

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