She serves a hopeful spinster perhaps (4-2-7)

I believe the answer is:

lady-in-waiting

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'she' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

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(Other definitions for lady-in-waiting that I've seen before include "Female attendant for a queen" , "Queen's personal attendant" , "Female attending royalty" , "Female attendant in a royal household" .)

I've seen this clue in The Globe and Mail.

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