The most common and most likely answer to this crossword clue is the 4 letter word FANG
I've seen this crossword clue in The Times
F
A
N
G
More clues leading to the result FANG
- Gasper — holding new one in the mouth
- Work hard to arrest northern poisoner
- Supporter meeting end of dog’s tooth
- Enthusiast on the spot
- Canine in a fit of anger
- Smoke to envelop new vampire weapon?
- Tooth extractor? Surgeon, at heart
- Irritation surrounding new tooth
- Intermittently flaunt gold front tooth
- Something in the mouth, new smoke rings
Buddy explains!

The straight part
We know that "one in the mouth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in the mouth = fang.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 FANG mean?
FANG noun- A long, pointed canine tooth used for biting and tearing flesh
- (in snakes) a long pointed tooth for injecting venom
- To strike or attack with the fangs.
- To enable to catch or tear; to furnish with fangs.
- To catch, capture; seize; grip; clutch; lay hold of.
- To take; receive with assent; accept.
- (as a guest) To receive with hospitality; welcome.
- (a thing given or imposed) To receive.
- To receive or adopt into spiritual relation, as in baptism; be godfather or godmother to.
- A grasping; capture; the act or power of seizing; hold.
- That which is seized or carried off; booty; spoils; stolen goods.
- Any projection, catch, shoot, or other thing by which hold is taken; a prehensile part or organ.
- A channel cut in the rock, or a pipe of wood, used for conveying air.
- (in the plural) Cage-shuts.
- The coil or bend of a rope; (by extension) a noose; a trap.
- The valve of a pump box.
- To supply (a pump) with the water necessary for it to operate.