The most common and most likely answer to this crossword clue is the 9 letter word CONTINUUM
I've seen this crossword clue in The Daily Telegraph
C
O
N
T
I
N
U
U
M
Buddy explains!
The straight part
We know that "sequence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sequence = continuum.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 CONTINUUM mean?
CONTINUUM noun- A continuous series or whole, no part of which is noticeably different from its adjacent parts, although the ends or extremes of it are very different from each other.
- A continuous extent.
- The set of real numbers; more generally, any compact connected metric space.
- A touch-sensitive strip, similar to an electronic standard musical keyboard, except that the note steps are 1/100 of a semitone, and so are not separately marked.