Singing girl accompanying Heather (9)
I believe the answer is:
carolling
'singing' is the definition.
(I have seen 'Seasonal singing ' mean 'carolling' so perhaps 'singing' could also mean 'carolling')
'girl accompanying heather' is the wordplay.
'girl' becomes 'carol' (woman's name).
'accompanying' is a charade indicator (letters next to each other).
'heather' becomes 'ling' (synonyms).
'carol'+'ling'='CAROLLING'
(Other definitions for carolling that I've seen before include "A Christmas activity" , "Singing at 7 across" , "Singing Christmas hymns" , "Seasonal singing" .)