The most common and most likely answer to this crossword clue is the 3 plus 4 letter word HAT TREE
I've seen this crossword clue in Independent
H
A
T
T
R
E
E
Buddy explains!
The straight part
We know that "a stand for 5 and 12" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a stand for 5 and 12 = hat tree.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.