The most common and most likely answer to this crossword clue is the 6 letter word CHITON
I've seen this crossword clue in Financial Times
C
H
I
T
O
N
More clues leading to the result CHITON
Buddy explains!

The straight part
We know that "Tunic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tunic = chiton.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 CHITON mean?
CHITON noun- A loose woolen tunic worn by men and women in Ancient Greece.
- Any of various rock-clinging marine molluscs of the class Polyplacophora, including the genus Chiton.