Rock band covering mostly soft, sad music (7)
I believe the answer is:
requiem
'sad music' is the definition.
'Requiem' can be an answer for 'music' (Requiem is an example). I'm not certain of the 'sad' bit.
'rock band covering mostly soft' is the wordplay.
'rock band' becomes 'rem' (REM are a US rock band).
'covering' means one lot of letters goes inside another.
'mostly soft' becomes 'equi' (I can't justify this - if you can you should give a lot more credence to this answer).
'rem' going around 'equi' is 'REQUIEM'.
Can you help me to learn more?
(Other definitions for requiem that I've seen before include "shark?" , "RC mass for the dead" , "Funeral service" , "Hymn of mourning" , "Funeral music" .)