The most common and most likely answer to this crossword clue is the 11 letter word SUPERLATIVE
I've seen this crossword clue in The Daily Telegraph
S
U
P
E
R
L
A
T
I
V
E
Buddy explains!
The straight part
We know that "Best" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case best = superlative.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 SUPERLATIVE mean?
SUPERLATIVE noun- The highest extent or degree of something.
- (grammar) The form of an adjective that expresses which of several items has the highest degree of the quality expressed by the adjective; in English, formed by appending "-est" to the end of the adjective (for some short adjectives only) or putting "most" before it.
- An adjective used to praise something exceptional.
- Exceptionally good; of the highest quality; superb.
- (grammar) Of or relating to a superlative.