The most common and most likely answer to this crossword clue is the 9 letter word FOREARMED
I've seen this crossword clue in The Guardian
F
O
R
E
A
R
M
E
D
Buddy explains!

The straight part
We know that "Well-prepared" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case well-prepared = forearmed.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 FOREARMED mean?
FOREARMED verb- (sometimes figurative) To arm in preparation.
- (in combination) Having some specific type of forearm.