The most common and most likely answer to this crossword clue is the 9 letter word DECILITRE
I've seen this crossword clue in The Guardian
D
E
C
I
L
I
T
R
E
Buddy explains!
The straight part
We know that "sixth of a pint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sixth of a pint = decilitre.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 DECILITRE mean?
DECILITRE noun- An SI unit of fluid equal to 10−1 liters. Symbol: dl.