The most common and most likely answer to this crossword clue is the 6 plus 5 letter word CAUGHT SHORT
I've seen this crossword clue in The Guardian
C
A
U
G
H
T
S
H
O
R
T
Buddy explains!
The straight part
We know that "Needing to go at once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case needing to go at once = caught short.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.