Popular number puzzle (6)
I believe the answer is:
sudoku
'popular number puzzle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sudoku that I've seen before include "Japanese number puzzle" , "numbers in boxes" , "Puzzle involving the numbers 1 to 9" , "Numerical puzzle" , "Popular logic-based puzzle" .)