The most common and most likely answer to this crossword clue is the 4 plus 4 letter word ZERO HOUR
I've seen this crossword clue in The Guardian
Z
E
R
O
H
O
U
R
More clues leading to the result ZERO HOUR
- Time to attack controversial type of contract, incomplete
- Love alluring woman no end: time for action?
- Time to attack endlessly unfair work conditions
- Time to get going? Oh!
- It’s when to start, but nothing’s on time
- Oh! It’s time to go!
- A critical time for one to leave alluring woman with nothing on
- Oh, it’s time for action
- No time at all to attack?
- Scheduled time for military operation? 00:00
Buddy explains!
The straight part
We know that "It’s when to start" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s when to start = zero hour.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.