Hormone abuse endlessly popular (7)
I believe the answer is:
insulin
'hormone' is the definition.
(I know that insulin is a type of hormone)
'abuse endlessly popular' is the wordplay.
I cannot really understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for insulin that I've seen before include "Pancreatic hormone" , "Secretion, deficiency of which results in diabetes" , "Hormone controlling sugar supply" , "Hormone controlling sugar intake, used for diabetes" , "Hormone lacking in diabetic" .)