The most common and most likely answer to this crossword clue is the 2 plus 3 letter word ON THE
I've seen this crossword clue in Independent
O
N
T
H
E
More clues leading to the result ON THE
Buddy explains!

The straight part
We know that "A 21 (freemason)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a 21 (freemason) = on the.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.