It's a cello playing swing (9)
I believe the answer is:
oscillate
'swing' is the definition.
(I know that oscillate is a kind of swing)
'it's a cello playing' is the wordplay.
'playing' indicates an anagram.
'its'+'a'+'cello'='itsacello'
'itsacello' is an anagram of 'OSCILLATE'.
(Other definitions for oscillate that I've seen before include "Swing like a pendulum" , "Move or swing from side to side regularly" , "Move to and fro like a pendulum" , "seesaw" , "Swing back and forth" .)