The most common and most likely answer to this crossword clue is the 13 letter word COCK-A-LEEKIE
I've seen this crossword clue in The Times
C
O
C
K
-
A
-
L
E
E
K
I
E
Buddy explains!
The straight part
We know that "soup" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case soup = cock-a-leekie.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.
What does COCK-A-LEEKIE mean?
COCK-A-LEEKIE noun- A Scottish chicken soup boiled with leeks.