It's very quick for musicians and others in a river (6)
I believe the answer is:
presto
'it's very quick' is the definition.
'others in a river' is the wordplay.
'others' becomes 'rest' (eg the rest of a group).
'in' is an insertion indicator.
'a river' becomes 'Po' (Italian river).
'rest' going within 'po' is 'PRESTO'.
'for musicians and' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for presto that I've seen before include "at great speed..." , "In quick tempo (mus.)" , "rapidly" , "29 [ADAGIO]? On the contrary" , "(Musically) fast" .)