The only time one and two don't make three (6)
I believe the answer is:
twelve
'three' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for twelve that I've seen before include "Highest number on clock" , "six couples" , "'Apostles, jury and months (6)'" , "Lunchtime" , "Midday" .)