The most common and most likely answer to this crossword clue is the 6 letter word SNIPER
I've seen this crossword clue in The Guardian
S
N
I
P
E
R
More clues leading to the result SNIPER
- Sharpshooter frets about missing bullseye at the end
- One shooting bird by river
- Rifleman given piece of cake by Queen?
- Gunman runs through again from South
- One shoots game bird, right?
- One shot Kennedy as President, perhaps not Ted
- Standing on legs, hidden marksman
- Control’s back, seizing power one picks off the unwary
- Bird first to reveal concealed assailant
- Ace shot wading bird, then rook
Buddy explains!

The straight part
We know that "Ace shot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ace shot = sniper.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 SNIPER mean?
SNIPER noun- A person using long-range small arms for precise attacks from a concealed position.
- Any attacker using a non-contact weapon against a specific target from a concealed position.
- One who shoots from a concealed position.
- One who criticizes.
- (ice hockey slang) A player who specializes in scoring goals.
- A person or automated process set up by a person who or which attempts to win an online auction by placing a bid only seconds before the auction ends, leaving no time for other bidders to respond