Lead singer rehearses The Sound of Music (7)
I believe the answer is:
strains
'the sound of music' is the definition.
(strain can mean how a musical piece sounds)
'lead singer rehearses' is the wordplay.
'lead' says to take the initial letters (I've seen 'leading' mean this).
'rehearses' becomes 'trains'.
The initial letter of 'singer' is 's'.
's'+'trains'='STRAINS'
(Other definitions for strains that I've seen before include "Stresses and - -" , "Tries very hard" , "Melodies" .)