Stuart Goddard's alter ego in Spain becomes unbreakable (10)
I believe the answer is:
adamantine
'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" .)