Slight cut in hormone (7)
I believe the answer is:
insulin
'hormone' is the definition.
(insulin is a kind of hormone)
'slight cut in' is the wordplay.
'slight' becomes 'insult' (I've seen this before).
'cut' means to remove the last letter.
'insult' with its last letter removed is 'insul'.
'insul'+'in'='INSULIN'
(Other definitions for insulin that I've seen before include "Hormone for treating diabetes" , "Hormone involved in diabetes" , "Pancreatic hormone" , "It regulates glucose in the blood" , "Treatment for diabetes" .)