Dancer's song mostly about Ireland (9)
I believe the answer is:
ballerina
'dancer's' is the definition.
(ballerina is a kind of dancer)
'song mostly about ireland' is the wordplay.
'song mostly' becomes 'balla' (I am not sure about this - if you are sure you should believe this answer much more).
'about' indicates putting letters inside.
'ireland' becomes 'Erin' (poetic name for Ireland).
'balla' going around 'erin' is 'BALLERINA'.
Can you help me to learn more?
(Other definitions for ballerina that I've seen before include "She's a classical dancer" , "Female classical dancer" , "Danseuse" , "girl on point duty" , "She dances to a rain bell" .)