How the siren’s accompanying herself on the piano? (7)
I believe the answer is:
vamping
'herself on the piano?' is the definition.
I can't tell whether this definition defines the answer.
'how the siren's accompanying' is the wordplay.
'how' becomes 'ing' (I can't explain this - if you can you should believe this answer much more).
'the siren' becomes 'vamp' (I've seen this before).
'accompanying' is a charade indicator (letters next to each other).
'ing' put after 'vamp' is 'VAMPING'.
Can you help me to learn more?
(Another definition for vamping that I've seen is " Seducing and exploiting (colloq.)".)