The most common and most likely answer to this crossword clue is the 7 plus 4 plus 2 plus 7 letter word LESSONS WILL BE LEARNED
I've seen this crossword clue in The Guardian
L
E
S
S
O
N
S
W
I
L
L
B
E
L
E
A
R
N
E
D
Buddy explains!
The straight part
We know that "(1 down justifying why not)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case (1 down justifying why not) = lessons will be learned.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.