The most common and most likely answer to this crossword clue is the 12 letter word PREMEDITATED
I've seen this crossword clue in The Daily Telegraph
P
R
E
M
E
D
I
T
A
T
E
D
Buddy explains!

The straight part
We know that "deliberate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deliberate = premeditated.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 PREMEDITATED mean?
PREMEDITATED verb- To meditate, consider, or plan beforehand; to think about and revolve in the mind beforehand.
- Planned, considered or estimated in advance; deliberate.