The most common and most likely answer to this crossword clue is the 5 plus 6 letter word ISAAC NEWTON
I've seen this crossword clue in The Times
I
S
A
A
C
N
E
W
T
O
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 = isaac newton.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.