The most common and most likely answer to this crossword clue is the 13 plus 4 letter word FRAGMENTATION BOMB
I've seen this crossword clue in The Guardian
F
R
A
G
M
E
N
T
A
T
I
O
N
B
O
M
B
Buddy explains!

The straight part
We know that "grenade" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grenade = fragmentation 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.