Almost all numbers to sort out — that's about a hundred to decipher (10)
I believe the answer is:
'decipher' is the definition.
'almost all numbers to sort out that's about a hundred' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'to sort out' indicates an anagram.
'that's about' is an insertion indicator.
'a hundred' becomes 'c' (roman numeral).
'all' with its last letter taken away is 'al'.
'alnumbers' anagrammed gives 'unsramble'.
'unsramble' going around 'c' is 'UNSCRAMBLE'.
'to' acts as a link.
(Other definitions for unscramble that I've seen before include "Restore to a readable state" , "Restore to readability" .)