Last characters in text indicate uncertain number (3)
I believe the answer is:
ten
'number' is the definition.
(ten is a kind of number)
'last characters in text indicate uncertain' is the wordplay.
'last characters in' indicates one should take the final letters.
The final letters of 'text indicate uncertain' are 'TEN'.
(Other definitions for ten that I've seen before include "Base of decimal system" , "Roman numeral X" , "X-WORD" , "honour" , "PM's address, Downing St" .)