The most common and most likely answer to this crossword clue is the 6 plus 6 letter word DOCTOR JEKYLL
I've seen this crossword clue in Financial Times
D
O
C
T
O
R
J
E
K
Y
L
L
Buddy explains!
The straight part
We know that "Lab engineer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lab engineer = doctor jekyll.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.