Indigestion (9)
(Other definitions for heartburn that I've seen before include "You'll get feeling of discomfort" , "Chest pain" , "Form of indigestion" , "Disorder" , "Hot sensation in lower chest from indigestion" .)
'indigestion' is the definition.
(I know that indigestion can be written as dyspepsia)
This is all the clue.
(Other definitions for dyspepsia that I've seen before include "Indigestion" , "It was enough to make any man sick" , "Stomach problem" , "internal upset" , "digestive complaint" .)