How I am yours (5)

I believe the answer is:

truly

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'how' is the definition.
Both the definition and answer are adverbs. Perhaps you can see an association between them that I don't see?

I don't understand how the rest of the clue works.

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(Other definitions for truly that I've seen before include "Genuinely, properly" , "with accuracy" , "Verily" , "In reality" , "Indeed" .)

I've seen this clue in the Evening Standard.

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