The most common and most likely answer to this crossword clue is the 4 plus 8 letter word WELL GROUNDED
I've seen this crossword clue in Financial Times
W
E
L
L
G
R
O
U
N
D
E
D
Buddy explains!

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 WELL GROUNDED mean?
WELL GROUNDED adjective- (of a person) Having extensive knowledge of a subject
- (of an argument etc) Based on good judgment and solid reasoning