The most common and most likely answer to this crossword clue is the 3 letter word TUN
T
U
N
More clues leading to the result TUN
Buddy explains!

The straight part
We know that "Large cask" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large cask = tun.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 TUN mean?
TUN noun- A large cask; an oblong vessel bulging in the middle, like a pipe or puncheon, and girt with hoops; a wine cask.
- A fermenting vat.
- An old English measure of capacity for liquids, containing 252 wine gallons; equal to two pipes.
- A weight of 2,240 pounds.
- An indefinite large quantity.
- A drunkard.
- Any shell belonging to Tonna and allied genera.
- The cryptobiotic state of a tardigrade, when its metabolism is temporarily suspended.
- To put into tuns, or casks.
- A part of the ancient Maya Long Count Calendar system which corresponds to 18 winal cycles or 360 days.