After pain, mainly ill due to this heath plant (9)
I believe the answer is:
tormentil
'this heath plant' is the definition.
The definition suggests a singular noun which matches the answer.
'after pain mainly ill' is the wordplay.
'after' says to put letters next to each other.
'pain' becomes 'torment' (torment is a kind of pain).
'mainly' means to remove the last letter.
'ill' with its last letter taken away is 'il'.
'torment' put next to 'il' is 'TORMENTIL'.
'due to' acts as a link.
Can you help me to learn more?
(Other definitions for tormentil that I've seen before include "Medicine ingredient" , "medicinal plant" , "Plant used to cure" , "Low plant" , "perennial" .)