The most common and most likely answer to this crossword clue is the 9 plus 4 letter word SOMETHING ELSE
I've seen this crossword clue in The Guardian
S
O
M
E
T
H
I
N
G
E
L
S
E
More clues leading to the result SOMETHING ELSE
Buddy explains!
The straight part
We know that "Not that/great" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not that/great = something else.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.