The most common and most likely answer to this crossword clue is the 6 plus 2 plus 4 letter word SECOND TO NONE
I've seen this crossword clue in The Daily Telegraph
S
E
C
O
N
D
T
O
N
O
N
E
More clues leading to the result SECOND TO NONE
- Sister’s deputy reportedly without parallel
- Best moment on pitch outside
- Back heavyweight against a nonpareil
- Runs 101-200? One is the best!
- Support not about to be given by individual better than the rest
- Back heavyweight individual? That’s best
- Back with hundred and one — as good as any
- Supreme moment, mood filled with drama
- Another heavy weight, individual best
Buddy explains!
The straight part
We know that "That’s best" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s best = second to none.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.