The most common and most likely answer to this crossword clue is the 3 letter word HOG
I've seen this crossword clue in Financial Times
H
O
G
Buddy explains!
The straight part
We know that "glutton" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case glutton = hog.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 HOG mean?
HOG noun- Any animal belonging to the Suidae family of mammals, especially the pig, the warthog, and the boar.
- (specifically) An adult swine (contrasted with a pig, a young swine).
- A greedy person; one who refuses to share.
- A large motorcycle, particularly a Harley-Davidson.
- A young sheep that has not been shorn.
- A rough, flat scrubbing broom for scrubbing a ship's bottom under water.
- A device for mixing and stirring the pulp from which paper is made.
- A shilling coin; its value, 12 old pence.
- A tanner, a sixpence coin; its value.
- A half-crown coin; its value, 30 old pence.
- The effect of the middle of the hull of a ship rising while the ends droop
- To greedily take more than one's share, to take precedence at the expense of another or others.
- To clip the mane of a horse, making it short and bristly.
- To scrub with a hog, or scrubbing broom.
- To cause the keel of a ship to arch upwards (the opposite of sag).
- To process (bark, etc.) into hog fuel.
- A quahog (clam)