The most common and most likely answer to this crossword clue is the 8 letter word THICKISH
I've seen this crossword clue in The Sunday Telegraph
T
H
I
C
K
I
S
H
More clues leading to the result THICKISH
Buddy explains!

The straight part
We know that "rather crowded?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rather crowded? = thickish.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.