Crossword Genius

Going on twelve inches (5)

Ross

I believe the answer is:

afoot

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

'going on' is the definition.
(I've seen this in another clue)

'twelve inches' is the wordplay.
I cannot quite see how this works, but
'twelve inches' could be 'foot' (I've seen this before) and 'foot' is found within the answer.
A single letter 'a' remains 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 afoot that I've seen before include "Currently in progress" , "Progressing, in operation - like Sherlock Holmes's game" , "Going on" , "Astir" , "Brewing" .)

I've seen this clue in The Globe and Mail.
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