One offering treatment, the first to include music (9)
I believe the answer is:
therapist
'one offering treatment' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition. I also know that 'treatment' relates to this answer.
'the first to include music' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'include' indicates putting letters inside.
'music' becomes 'rap' (musical genre).
'the'+'ist'='theist'
'theist' placed around 'rap' is 'THERAPIST'.
Can you help me to learn more?
(Other definitions for therapist that I've seen before include "Help" , "Shrink" , "One treating mental disorders" , "One giving treatment" , "See 21" .)