The most common and most likely answer to this crossword clue is the 14 letter word CHARLESÂ DARWIN
I've seen this crossword clue in The Guardian
C
H
A
R
L
E
S
Â
D
A
R
W
I
N
Buddy explains!

The straight part
We know that "scientist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case scientist = charles darwin.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.