The most common and most likely answer to this crossword clue is the 3 plus 6 letter word GOT BEHIND
I've seen this crossword clue in The Guardian
G
O
T
B
E
H
I
N
D
Buddy explains!
The straight part
We know that "Failed to keep up/supported" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case failed to keep up/supported = got behind.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.