Crossword Genius

One's wanted to stay, heading right down in fact (11)

Ross

I believe the answer is:

desideratum

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

'fact' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'one's wanted to stay heading right down' is the wordplay.
I cannot really see how this works, but
'right' could be 'r' (abbreviation) and 'r' is found within the answer.
'down' could be 'eat' (I've seen this in other clues) and 'eat' is found in the leftover letters.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
This explanation may well be incorrect...

'in' is the link.

Can you help me to learn more?

(Other definitions for desideratum that I've seen before include "As requirement" , "Something much wanted" , "Essential" , "Something needed or wanted" , "Something much needed" .)

I've seen this clue in The Times.
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