Using the bow, in music (4)
I believe the answer is:
arco
'using the bow in music' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arco that I've seen before include "taking a bow" , "Played with the bow" , "which every violinist must grasp" , "bow for players" , "Musical direction" .)