The most common and most likely answer to this crossword clue is the 8 letter word CRACKING
I've seen this crossword clue in The Times
C
R
A
C
K
I
N
G
Buddy explains!
The straight part
We know that "Very fast/solving" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very fast/solving = cracking.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 CRACKING mean?
CRACKING verb- To form cracks.
- To break apart under pressure.
- To become debilitated by psychological pressure.
- To break down or yield, especially under interrogation or torture.
- To make a cracking sound.
- (of a voice) To change rapidly in register.
- (of a pubescent boy's voice) To alternate between high and low register in the process of eventually lowering.
- To make a sharply humorous comment.
- To make a crack or cracks in.
- To break open or crush to small pieces by impact or stress.
- To strike forcefully.
- To open slightly.
- To cause to yield under interrogation or other pressure. (Figurative)
- To solve a difficult problem. (Figurative, from cracking a nut.)
- To overcome a security system or a component.
- To cause to make a sharp sound.
- To tell (a joke).
- To break down (a complex molecule), especially with the application of heat: to pyrolyse.
- To circumvent software restrictions such as regional coding or time limits.
- To open a canned beverage, or any packaged drink or food.
- To brag, boast.
- To be ruined or impaired; to fail.
- The thermal decomposition of a substance, especially that of crude petroleum in order to produce petrol / gasoline.
- The formation of cracks on a surface.
- The production of a crack sound.
- Great.
- Enjoyable.
- Very, usually associated with praise.