Ballet smaller, certainly, keeping record concealed (3,9)

'ballet' is the definition.
(ballet by Frederic Chopin)

'smaller certainly keeping record concealed' is the wordplay.
'smaller' becomes 'less' (less great in size).
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'keeping' means one lot of letters goes inside another.
'record' becomes 'LP' (abbreviation for long play record).
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'lp'+'hid'='lphid'
'yes' enclosing 'lphid' is 'ylphides'.
'less'+'ylphides'='LES SYLPHIDES'