One is playing music, perhaps (5)
I believe the answer is:
noise
'music perhaps' is the definition.
(music is sound or noise)
'one is playing' is the wordplay.
'playing' is an anagram indicator.
'one'+'is'='oneis'
'oneis' is an anagram of 'NOISE'.
(Other definitions for noise that I've seen before include "Virgil's birthplace" , "Unpleasant sounds" , "Unstructured sound" , "Clamour" , "Clatter" .)