Black rocks rolling around inside viola, occasionally (4)
I believe the answer is:
coal
'black rocks' is the definition.
(coal is black)
'rolling around inside viola occasionally' is the wordplay.
'rolling around' is a reversal indicator.
'inside' indicates the answer is hidden within the clue.
'laoc' can be found hidden inside 'viola occasionally'.
'laoc' reversed gives 'COAL'.
(Other definitions for coal that I've seen before include "Carbon fuel" , "Carbonized vegetable matter" , "Solid fuel" , "Black mineral used for burning" , "Three domestic fuels" .)