Deer crossing English stream finds pain (9)
I believe the answer is:
'pain' is the definition.
(I know that heartburn is a type of pain)
'deer crossing english stream' is the wordplay.
'deer' becomes 'hart' (hart is a kind of deer).
'crossing' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'stream' becomes 'burn' (a burn is a small stream).
'hart' enclosing 'e' is 'heart'.
'finds' acts as a link.
(Other definitions for heartburn that I've seen before include "Hot sensation in lower chest from indigestion" , "Acid indigestion" , "Severe indigestion" , "Disorder" , "minor ailment" .)