The most common and most likely answer to this crossword clue is the 10 plus 4 letter word EVAPORATED MILK
I've seen this crossword clue in Independent
E
V
A
P
O
R
A
T
E
D
M
I
L
K
More clues leading to the result EVAPORATED MILK
Buddy explains!
The straight part
We know that "thick liquid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thick liquid = evaporated milk.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.