Strictly orthodox Jew can prove who he is (5)

I believe the answer is:

hasid

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'orthodox jew can prove who he is' is the definition.
'hasid' can be an answer for 'orthodox jew' (I've seen this before). I'm not sure about the rest of the definition.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for hasid that I've seen before include "Orthodox believer" , "Orthodox Jew" , "Pious Jew" , "Member of Jewish sect" .)

I've seen this clue in The Guardian.

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