The most common and most likely answer to this crossword clue is the 11 letter word VASOPRESSIN
V
A
S
O
P
R
E
S
S
I
N
Buddy explains!

The straight part
We know that "hormone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hormone = vasopressin.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.
Crossword Answer definitions
VASOPRESSIN noun- An antidiuretic hormone secreted by the pituitary gland.