Do a song about Welsh jazz music (5)
I believe the answer is:
swing
'jazz music' is the definition.
(I know that swing is a type of jazz music)
'do a song about welsh' is the wordplay.
'do a song' becomes 'sing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'welsh' becomes 'W' (abbreviation).
'sing' enclosing 'w' is 'SWING'.
Can you help me to learn more?
(Other definitions for swing that I've seen before include "Influence" , "Change opinion" , "Big band jazz" , "Big Band Sound" , "Jaunty rhythm in music" .)