Going on twelve inches (5)
I believe the answer is:
afoot
'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" .)