The most common and most likely answer to this crossword clue is the 4 letter word FOAM
I've seen this crossword clue in The Guardian
F
O
A
M
More clues leading to the result FOAM
Buddy explains!
The straight part
We know that "It emerges from opening a 12, initially" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it emerges from opening a 12, initially = foam.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 FOAM mean?
FOAM noun- A substance composed of a large collection of bubbles or their solidified remains.
- A substance formed by trapping pockets of gas in a liquid or solid.
- (by extension) Sea foam; the sea.
- Fury.
- To form or emit foam.
- To spew saliva as foam, to foam at the mouth.