Lunatic’s helmet? (8)

I believe the answer is:

headcase

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But I can't explain it at all! Could you help me understand?

(Other definitions for headcase that I've seen before include "Crazy person (slang)" , "Unstable person" , "fruitcake" , "Crazy chap" , "One must be mad" .)

I've seen this clue in The Hindu.

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