The most common and most likely answer to this crossword clue is the 4 plus 4 plus 3 plus 6 letter word THAT SHIP HAS SAILED
I've seen this crossword clue in The Guardian
T
H
A
T
S
H
I
P
H
A
S
S
A
I
L
E
D
Buddy explains!
The straight part
We know that "too late!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case too late! = that ship has sailed.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.