Never-ending scales, see, when playing (9)
I believe the answer is:
ceaseless
'never-ending' is the definition.
(I've seen this before)
'scales see when playing' is the wordplay.
'when playing' is an anagram indicator.
'scales'+'see'='scalessee'
'scalessee' is an anagram of 'CEASELESS'.
(Other definitions for ceaseless that I've seen before include "Continual" , "Without end" , "Non-stop" , "Continuous" , "Never-ending" .)