This number is possibly over ten! (3)
I believe the answer is:
six
'this number' is the definition.
(six is a kind of number)
'is possibly over ten' is the wordplay.
'possibly' is an anagram indicator.
'over' says to put letters next to each other.
'ten' becomes 'X' (Roman numeral).
'is' with letters rearranged gives 'si'.
'si'+'x'='SIX'
(Other definitions for six that I've seen before include "Number of US moon landings" , "Half-dozen" , "larger boundary" , "Half a dozen" , "Knock for . . . - surprise or defeat" .)