Crossword Genius

One of those in the house synonymous with a diet? (15)

Ross

I believe the answer is:

parliamentarian

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

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

'those in the house synonymous with a diet?' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'diet?' could be 'parliament' (I've seen this before) and 'parliament' is found within the answer.
This may be the basis of clue (or it may be nonsense).

'of' is the link.

Can you help me to learn more?

(Other definitions for parliamentarian that I've seen before include "legislator" , "Eg, an MP" , "politician" , "elected representative?" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.
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