The most common and most likely answer to this crossword clue is the 12 letter word PERIODONTICS
I've seen this crossword clue in The Daily Telegraph
P
E
R
I
O
D
O
N
T
I
C
S
Buddy explains!
The straight part
We know that "part of dentistry" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of dentistry = periodontics.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 PERIODONTICS mean?
PERIODONTICS noun- The study of supporting structures of teeth—gums, alveolar bone, cementum, and the periodontal ligament—and diseases and conditions that affect them.