The most common and most likely answer to this crossword clue is the 4 letter word FRAG
F
R
A
G
More clues leading to the result FRAG
Buddy explains!

The straight part
We know that "Zap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case zap = frag.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 FRAG mean?
FRAG noun- A fragmentation grenade.
- A successful kill in a deathmatch game.
- To deliberately kill (one's superior officer) with a fragmentation grenade.
- To hit with the explosion of a fragmentation grenade.
- To kill.