The most common and most likely answer to this crossword clue is the 8 plus 5 letter word COMPLETE IDIOT
I've seen this crossword clue in The Guardian
C
O
M
P
L
E
T
E
I
D
I
O
T
Buddy explains!
The straight part
We know that "Total fool" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case total fool = complete idiot.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.