The most common and most likely answer to this crossword clue is the 9 plus 6 letter word ENDOCRINE GLANDS
I've seen this crossword clue in The Guardian
E
N
D
O
C
R
I
N
E
G
L
A
N
D
S
Buddy explains!
The straight part
We know that "blood suppliers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blood suppliers = endocrine glands.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.