The most common and most likely answer to this crossword clue is the 12 letter word BLACKBALLING
I've seen this crossword clue in The Times
B
L
A
C
K
B
A
L
L
I
N
G
Buddy explains!
The straight part
We know that "Exclusion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case exclusion = blackballing.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 BLACKBALLING mean?
BLACKBALLING verb- To vote against, especially in an exclusive organization.
- To ostracize.
- An instance, or action, of a person being blackballed