The most common and most likely answer to this crossword clue is the 4 plus 4 letter word NAIL BOMB
I've seen this crossword clue in The Guardian
N
A
I
L
B
O
M
B
More clues leading to the result NAIL BOMB
Buddy explains!
The straight part
We know that "form of explosive device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case form of explosive device = nail bomb.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.