The most common and most likely answer to this crossword clue is the 6 letter word FINALS
I've seen this crossword clue in The Daily Telegraph
F
I
N
A
L
S
Buddy explains!
The straight part
We know that "Last of tests" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case last of tests = finals.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 FINALS mean?
FINALS noun- A final examination; a test or examination given at the end of a term or class; the test that concludes a class.
- The last round, game or match in a contest, after which the winner is determined.
- A contest that narrows a field of contestants (finalists) to ranked positions, usually in numbered places (1st place/prize, 2nd place/prize, etc.) or a winner and numbered runners-up (1st runner-up, etc.).
- The final part of a syllable, the combination of medial and rime in phonetics and phonology.
- The tonic or keynote of a Gregorian mode, and hence the final note of any conventional melody played in that mode.