Lied about trendy lass’s informal music-making? (9)
I believe the answer is:
singalong
'informal music-making?' is the definition.
(impromptu singing session)
'lied about trendy lass's' is the wordplay.
'lied' becomes 'song' ('Lied' is German for song).
'about' indicates putting letters inside.
'trendy' becomes 'in' ('in' can mean fashionable).
'lass' becomes 'gal' (both can mean a girl).
'in'+'gal'='ingal'
'song' placed around 'ingal' is 'SINGALONG'.
(Other definitions for singalong that I've seen before include "Informal group musical activity" , "in which everyone joins in?" , "people making music together" , "chant" , "social occasion" .)