The most common and most likely answer to this crossword clue is the 7 plus 7 letter word HOLIDAY TRAFFIC
I've seen this crossword clue in The Guardian
H
O
L
I
D
A
Y
T
R
A
F
F
I
C
Buddy explains!
The straight part
We know that "cause of congestion?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cause of congestion? = holiday traffic.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.