For musicians, very quick break in river (6)
I believe the answer is:
presto
'for musicians very quick' is the definition.
I can't tell whether this definition defines the answer.
'break in river' is the wordplay.
'break' becomes 'rest' (a rest is taking a break).
'in' indicates putting letters inside.
'river' becomes 'Po' (Italian river).
'rest' placed into 'po' is 'PRESTO'.
Can you help me to learn more?
(Other definitions for presto that I've seen before include "Hey ..., said the magician" , "(Musically) fast" , "Very fast (mus.)" , "hey......,cries the magician" , "Hey, quickly for a magician" .)