Insulin, say, moorhen might produce (7)
I believe the answer is:
hormone
'insulin' is the definition.
(thesaurus)
'moorhen might produce' is the wordplay.
'might produce' indicates anagramming the letters (I've seen this in other clues).
'moorhen' anagrammed gives 'HORMONE'.
'say' is the link.
(Other definitions for hormone that I've seen before include "Substance like insulin" , "Secretion transported in the blood" , "Stimulating bodily secretion" , "A body chemical" , "Substance that stimulates cells in moorhen" .)