The most common and most likely answer to this crossword clue is the 7 plus 5 letter word CUSTOMS UNION
I've seen this crossword clue in Financial Times
C
U
S
T
O
M
S
U
N
I
O
N
Buddy explains!
The straight part
We know that "feature of 8" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case feature of 8 = customs union.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.