Twice this clue's number (3)
I believe the answer is:
ten
'twice this clue's number' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ten that I've seen before include "Five times two" , "Very young" , "X-WORD" , "Dudley Moore/Bo Derek film" , "Roman numeral X" .)