Number of square feet in a square yard (4)
I believe the answer is:
nine
'number of square feet in a square yard' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nine that I've seen before include "Number of Muses and days in a Novena" , "Number of feline existences" , "Square number" , "3x3" , "Figure" .)