The most common and most likely answer to this crossword clue is the 3 letter word KEG
K
E
G
Buddy explains!
The straight part
We know that "This tapped: slake gallons" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this tapped: slake gallons = keg.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 KEG mean?
KEG noun- A round, traditionally wooden container of lesser capacity than a barrel, often used to store beer.
- To store in a keg.