The most common and most likely answer to this crossword clue is the 3 plus 2 plus 4 letter word ALL AT ONCE
I've seen this crossword clue in Independent
A
L
L
A
T
O
N
C
E
More clues leading to the result ALL AT ONCE
- Simultaneously everyone agreed to accept Conservative
- Suddenly, everyone agreed about carbon
- Without warning Arthur initially, Lancelot is false
- Without notice, one keeps just some back
- Suddenly everyone agreed to go round clubs
- Suddenly tall ocean waves
- Everyone agreed about carbon, suddenly
- Expert garners unbelievable return working so quickly
- How Wordsworth saw a crowd in a high-rise long ago
- Suddenly cat left alone goes berserk
Buddy explains!

The straight part
We know that "Suddenly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suddenly = all at once.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.