Crossword Genius

So I’m building a mansion with her (3)

Ross

I believe the answer is:

ann

I'm a little stuck... Click here to teach me more about this clue!

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

'so i'm building a mansion' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'with' acts as a link.

Can you help me to learn more?

(Other definitions for ann that I've seen before include "eg Miss Widdecombe" , "- - Jones, 1969 Wimbledon champion" , "woman" , "Ms Widdecombe" , "Girl's name" .)

I've seen this clue in the Evening Standard.
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