The most common and most likely answer to this crossword clue is the 4 plus 4 letter word FIVE SETS
I've seen this crossword clue in Financial Times
F
I
V
E
S
E
T
S
Buddy explains!
The straight part
We know that "a test for 21 at 1 across" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a test for 21 at 1 across = five sets.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.