Crossword Genius

Is a high one infantile? (5)

Ross

I believe the answer is:

chair

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

'is a' is the definition.
I can't tell whether this definition defines the answer.

'high one infantile?' is the wordplay.
I cannot quite understand how this works, but
'high' could be 'h' (abbreviation) and 'h' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
The remaining letters 'car' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for chair that I've seen before include "Item of furniture (which can be addressed!)" , "A committee's most important seat" , "Single seat" , "Lead" , "Official position of authority" .)

I've seen this clue in the Evening Standard.
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