Number it: a 100 written up as symbols (8)
I believe the answer is:
notation
'symbols' is the definition.
(I've seen this before)
'number it a 100 written up' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
't00' becomes 'ton' (I've seen this in other clues).
'written up' shows that the letters should be reversed in order.
'no'+'it'+'a'+'ton'='noitaton'
'noitaton' backwards is 'NOTATION'.
'as' is the link.
(Other definitions for notation that I've seen before include "Set of representative symbols" , "System of signs or symbols" , "Written comment" , "Set of symbols representing quantity etc." , "A comment added to a text" .)