Healthy head on more revolting secretion (7)
I believe the answer is:
hormone
'secretion' is the definition.
(I know that hormone is a type of secretion)
'healthy head on more revolting' is the wordplay.
'head' says to take the initial letters.
'revolting' is an anagram indicator.
The first letter of 'healthy' is 'h'.
'on'+'more'='onmore'
'onmore' with letters rearranged gives 'ormone'.
'h'+'ormone'='HORMONE'
(Other definitions for hormone that I've seen before include "Substance like insulin" , "Growth secretion" , "Glandular secretion with stimulating properties" , "Stimuating substance produced by body" , "Substance that stimulates cells in moorhen" .)