One who talks one round to composing some music (8)
I believe the answer is:
oratorio
'some music' is the definition.
(oratorio is a kind of music)
'one who talks one round to composing' is the wordplay.
'one who talks' becomes 'orator' (I can't explain this - if you can you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'round' becomes 'o' (round can mean a round object).
'to composing' means one lot of letters go next to another.
'orator'+'i'+'o'='ORATORIO'
Can you help me to learn more?
(Other definitions for oratorio that I've seen before include "Work performed by choir" , "eg Handel's 'Messiah'" , "Religious musical drama, unstaged" , "religious music" , "Sacred, dramatic music" .)