The most common and most likely answer to this crossword clue is the 8 plus 5 letter word ACQUIRED TASTE
I've seen this crossword clue in The Daily Telegraph
A
C
Q
U
I
R
E
D
T
A
S
T
E
More clues leading to the result ACQUIRED TASTE
Buddy explains!
The straight part
We know that "such a thing as Marmite" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case such a thing as marmite = acquired taste.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.