Book written backwards by one fool combined symbols (8)
I believe the answer is:
'combined symbols' is the definition.
(symbol made up of combined keyboard characters)
'book written backwards by one fool' is the wordplay.
'book' becomes 'tome' (a tome is a book).
'written backwards' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'fool' becomes 'con' (I've seen this in another clue).
'tome' backwards is 'emot'.
(Other definitions for emoticon that I've seen before include "Wordless expression" , "Representation of a facial expression in electronic messages" , "Textual symbol" , "symbol of modern communication" , "Smiley characters" .)