Day without commencement of night music (5)
I believe the answer is:
tunes
'day without commencement of night music' is the definition.
I can't tell whether this definition defines the answer.
'day without commencement of night' is the wordplay.
'day' becomes 'tues' (abbreviation for Tuesday).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'commencement of' indicates taking the first letters.
The first letter of 'night' is 'n'.
'tues' enclosing 'n' is 'TUNES'.
Can you help me to learn more?
(Another definition for tunes that I've seen is " Airs, melodies".)