Pain from lung area possibly including his heart (9)
I believe the answer is:
neuralgia
'pain' is the definition.
(neuralgia is a kind of pain)
'lung area possibly including his heart' is the wordplay.
'possibly' is an anagram indicator.
'including' means one lot of letters goes inside another.
'heart' says to take the centre.
The centre of 'his' is 'i'.
'lung'+'area'='lungarea'
'lungarea' anagrammed gives 'neuralga'.
'neuralga' placed around 'i' is 'NEURALGIA'.
'from' acts as a link.
(Other definitions for neuralgia that I've seen before include "Severe pain through damaged nerves" , "Sharp pain extending along nerves" , "sort of pain" , "Pains in the neck" , "Source of pain" .)