Number of self-contained flat (6)

I believe the answer is:

eleven

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

'number' is the definition.
(eleven is a kind of number)

'self-contained flat' is the wordplay.
'self contained' becomes 'el' (I am not sure about this - if you are sure you should believe this answer much more).
'flat' becomes 'even' (level).
'el'+'even'='ELEVEN'

'of' is the link.

Can you help me to learn more?

(Other definitions for eleven that I've seen before include "Age to start secondary education" , "Number, on a team maybe" , "Football side" , "Number on a side (6)" , "Players" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.