The most common and most likely answer to this crossword clue is the 6 plus 7 letter word CRUISE MISSILE
I've seen this crossword clue in The Daily Telegraph
C
R
U
I
S
E
M
I
S
S
I
L
E
Buddy explains!
The straight part
We know that "powerful weapon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case powerful weapon = cruise missile.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.