Hormone is in lung, endlessly circulating (7)
I believe the answer is:
insulin
'hormone' is the definition.
(insulin is a kind of hormone)
'is in lung endlessly circulating' is the wordplay.
'endlessly' means to remove the last letter.
'circulating' is an anagram indicator.
'is'+'in'+'lung'='isinlung'
'isinlung' with its last letter removed is 'isinlun'.
'isinlun' anagrammed gives 'INSULIN'.
(Other definitions for insulin that I've seen before include "Hormone that controls the body's sugar absorption" , "Secretion, deficiency of which results in diabetes" , "Hormone substance injected by diabetics" , "Hormone involved in diabetes" , "Hormone - nil in us (anag)" .)