The most common and most likely answer to this crossword clue is the 12 letter word DOUBLE-QUICK
D
O
U
B
L
E
-
Q
U
I
C
K
Buddy explains!

The straight part
We know that "Superfast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case superfast = double-quick.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.
What does DOUBLE-QUICK mean?
DOUBLE-QUICK noun- The double time, a marching cadence (pace) approaching running speed.
- To double-time, march (or rather run) at the above speedy pace.
- Very quickly; at once; on the double.