Hormone used by Linus, possibly, at home (7)
I believe the answer is:
insulin
'hormone' is the definition.
(I know that insulin is a type of hormone)
'by linus possibly at home' is the wordplay.
'by' says to put letters next to each other.
'possibly' indicates an anagram.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'linus' is an anagram of 'insul'.
'insul'+'in' is 'INSULIN'.
'used' acts as a link.
(Other definitions for insulin that I've seen before include "Treatment for diabetic" , "Treatment for diabetes" , "Hormone controlling sugar supply" , "Hormone regulating the amount of glucose in the blood" , "blood sugar regulated by this" .)