The most common and most likely answer to this crossword clue is the 5 plus 6 letter word BLOOD VESSEL
I've seen this crossword clue in The Daily Telegraph
B
L
O
O
D
V
E
S
S
E
L
More clues leading to the result BLOOD VESSEL
Buddy explains!

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.