Simple song: could it be any louder? (9)
I believe the answer is:
roundelay
'simple song' is the definition.
(I know that roundelay is a type of simple song)
'could it be any louder?' is the wordplay.
'could it be' indicates anagramming the letters.
'any'+'louder'='anylouder'
'anylouder' is an anagram of 'ROUNDELAY'.
(Other definitions for roundelay that I've seen before include "Circle dance; simple song" , "Slow, circling medieval dance" , "Short, simple song" , "Slow dance - song with refrain" , "Slow, medieval dance, in a circle" .)