Medication for the heart is not to be compounded in it (9)
I believe the answer is:
cortisone
'medication for' is the definition.
The definition suggests a singular noun which matches the answer.
'heart is not to be compounded in it' is the wordplay.
'heart' becomes 'core' (synonyms).
'to be compounded' is an anagram indicator.
'in it' indicates putting letters inside.
'is'+'not'='isnot'
'isnot' is an anagram of 'tison'.
'core' enclosing 'tison' is 'CORTISONE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for cortisone that I've seen before include "hormone used in medicine!" , "Steroid" , "Drug" , "anti-inflammatory one" .)