With the bow, musically (4)
I believe the answer is:
arco
'with the bow musically' 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 "bow for players" , "Musical direction" , "which every violinist must grasp" , "Played with the bow" , "taking a bow" .)