Showcase at low volume in underworld ballad (7)
I believe the answer is:
display
'showcase' is the definition.
(I've seen this before)
'at low volume in underworld ballad' is the wordplay.
'at low volume' becomes 'p' (musical symbol for 'piano', ie softly or quiet).
'in' is an insertion indicator.
'underworld' becomes 'dis' (underworld in classical mythology).
'ballad' becomes 'lay' (synonyms).
'dis'+'lay'='dislay'
'p' inserted inside 'dislay' is 'DISPLAY'.
(Other definitions for display that I've seen before include "Exhibit, show" , "Show or arrange" , "Show to the public" , "Place on show" , "Exhibition" .)