Help rejected by old heretic? That's symptomatic! (10)
I believe the answer is:
diagnostic
'symptomatic' is the definition.
('diagnostic' can be a synonym of 'symptomatic')
'help rejected by old heretic?' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'rejected' is a reversal indicator.
'by' means one lot of letters go next to another.
'old heretic?' becomes 'gnostic'.
'aid' written backwards gives 'dia'.
'dia'+'gnostic'='DIAGNOSTIC'
'that's' acts as a link.
(Other definitions for diagnostic that I've seen before include "Indicating the nature of an illness" , "Computer program" , "Analytical" , "Distinctive symptom or characteristic (of a disease)" .)