The most common and most likely answer to this crossword clue is the 12 letter word VOCIFERATING
I've seen this crossword clue in The Daily Telegraph
V
O
C
I
F
E
R
A
T
I
N
G
Buddy explains!

The straight part
We know that "shouting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shouting = vociferating.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.
Crossword Answer definitions
VOCIFERATING verb- To cry out with vehemence
- To utter with a loud voice; to shout out.