Square root of nine (5)
I believe the answer is:
three
'square root of nine' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for three that I've seen before include "Sixth root of seven hundred and twenty nine" , "Number that's a crowd" , "Prime number" , "So many 12s today" , "Amount in a trio" .)