One may offer a fortune for a house (10)

I believe the answer is:

astrologer

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for astrologer that I've seen before include "Studier of the supposed influence of stars" , "One who reads the stars" , "Compiler of star charts" , "Fortune-teller using zodiac" , "Horoscopist" .)

I've seen this clue in The Times and The Sunday Times.

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