Playing the viola is wasting time aimlessly (8)
I believe the answer is:
fiddling
'playing' is the definition.
('fiddle' can be a synonym of 'play')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fiddling that I've seen before include "wasting time?" , "Such swindles may be" , "What Nero was doing while Rome burned" , "petty" , "Cooking the books" .)