The most common and most likely answer to this crossword clue is the 2 plus 3 plus 2 plus 4 plus 4 letter word GO OFF AT HALF COCK
I've seen this crossword clue in The Guardian
G
O
O
F
F
A
T
H
A
L
F
C
O
C
K
Buddy explains!
The straight part
We know that "start ill-prepared" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case start ill-prepared = go off at half cock.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.