Vulnerable citizen (7)

I believe the answer is:

subject

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This might be a double definition.

'vulnerable' is the first definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?

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

Can you help me to learn more?

(Other definitions for subject that I've seen before include "Division of curriculum" , "UK citizen?" , "Put down" , "Expose; field of study" , "guinea pig" .)

I've seen this clue in The Telegraph.

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