The most common and most likely answer to this crossword clue is the 12 letter word RIBONUCLEASE
I've seen this crossword clue in The Times
R
I
B
O
N
U
C
L
E
A
S
E
Buddy explains!
The straight part
We know that "enzyme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case enzyme = ribonuclease.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 RIBONUCLEASE mean?
RIBONUCLEASE noun- Any of a group of enzymes that catalyze the hydrolysis of ribonucleic acid.