The most common and most likely answer to this crossword clue is the 3 plus 4 plus 5 letter word THE LAST STRAW
I've seen this crossword clue in The Daily Telegraph
T
H
E
L
A
S
T
S
T
R
A
W
Buddy explains!
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.