Protein hormone (7)
I believe the answer is:
insulin
'protein hormone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for insulin that I've seen before include "Hormone produced in the pancreas to control body sugar" , "Hormone that regulates glucose levels" , "Hormone lacking in diabetic" , "Hormone controlling sugar supply" , "Diabetes drug" .)