The most common and most likely answer to this crossword clue is the 6 letter word TETRAD
I've seen this crossword clue in The Sunday Telegraph
T
E
T
R
A
D
Buddy explains!
The straight part
We know that "quartet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quartet = tetrad.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 TETRAD mean?
TETRAD noun- A group of four things.
- Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
- A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.
- A unit of land area of two by two (that is, four) square kilometres.
- A tetravalent atom or radical.
- A group of four basis vectors for a four-dimensional manifold in differential geometry.