Crossword Genius

Bench on one Concorde's left wing? Not so fast (8)

Ross

I believe the answer is:

subsonic

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'not so fast' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'bench on one concorde's left wing?' is the wordplay.
I cannot really understand how this works, but
'on' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for subsonic that I've seen before include "At a speed less than that of sound" , "Of speed lower than that of sound" , "Less speedy than sound" , "Travelling at less than Mach 1" , "Such low rates for Jumbos" .)

I've seen this clue in The Independent.
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