It regulates glucose in the blood (7)
I believe the answer is:
insulin
'it regulates glucose in the blood' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for insulin that I've seen before include "Regulator of blood sugar" , "Hormone controlling sugar intake, used for diabetes" , "Hormone that regulates glucose levels" , "Hormone substance injected by diabetics" , "Hormone important to diabetics" .)