The most common and most likely answer to this crossword clue is the 5 letter word BUSHY
I've seen this crossword clue in The Guardian
B
U
S
H
Y
Buddy explains!
The straight part
We know that "Thick" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thick = bushy.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 BUSHY mean?
BUSHY adjective- Like a bush in having many widely spread branches.
- Growing thickly.
- (Racial slur) Derogatory word used to refer to Afro-textured Hair.