The most common and most likely answer to this crossword clue is the 6 letter word OVALÂ O
O
V
A
L
Â
O
Buddy explains!

The straight part
We know that "closed 12 Ac." is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case closed 12 ac. = oval o.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.