The most common and most likely answer to this crossword clue is the 8 letter word INFORMED
I've seen this crossword clue in Financial Times
I
N
F
O
R
M
E
D
Buddy explains!

The straight part
We know that "Knowledgeable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knowledgeable = informed.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 INFORMED mean?
INFORMED verb- To instruct, train (usually in matters of knowledge).
- To communicate knowledge to.
- To impart information or knowledge.
- To act as an informer; denounce.
- To give form or character to; to inspire (with a given quality); to affect, influence (with a pervading principle, idea etc.).
- To make known, wisely and/or knowledgeably.
- To direct, guide.
- To take form; to become visible or manifest; to appear.
- Instructed; having knowledge of a fact or area of education.
- Based on knowledge; founded on due understanding of a situation.
- Created, given form.
- Unformed or ill-formed; deformed; shapeless
- Not included within the figures of any of the ancient constellations.