The most common and most likely answer to this crossword clue is the 4 letter word BARM
I've seen this crossword clue in The Times
B
A
R
M
More clues leading to the result BARM
Buddy explains!

The straight part
We know that "froth on liquor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case froth on liquor = barm.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 BARM mean?
BARM noun- (obsolete outside dialectal) Bosom, lap.
- Foam rising upon beer, or other malt liquors, when fermenting, and used as leaven in making bread and in brewing; yeast.
- A small, flat, round individual loaf or roll of bread.
- To spurge; foam