Band, very quiet otherwise, introducing piece of music at appropriate time (9)
I believe the answer is:
opportune
'at appropriate time' is the definition.
(at the right opportunity)
'band very quiet otherwise introducing piece of music' is the wordplay.
'band' becomes 'o' (I've seen this in other clues).
'very quiet' becomes 'pp' (musical abbreviation for pianissimo).
'otherwise' becomes 'or'.
'introducing' says to put letters next to each other (some letters go before others).
'piece of music' becomes 'tune' (a tune is a piece of music).
'o'+'pp'+'or'+'tune'='OPPORTUNE'
(Other definitions for opportune that I've seen before include "Especially convenient" , "Timely, advantageous moment" , "Timely and convenient" , "Well-timed" , "fit" .)