Crossword Genius

Stuart Goddard's alter ego in Spain becomes unbreakable (10)

Ross

I believe the answer is:

adamantine

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'unbreakable' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for adamantine that I've seen before include "firm" , "very strong" , "Unbreakable" , "impossible to break into" , "attractive" .)

I've seen this clue in The Independent.
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