Suddenly others plunged into Italian river (6)
I believe the answer is:
presto
'suddenly' is the definition.
'others plunged into italian river' is the wordplay.
'others' becomes 'rest' (I've seen this before).
'plunged into' is an insertion indicator.
'italian river' becomes 'po' (river in Italy).
'rest' placed inside 'po' is 'PRESTO'.
(Other definitions for presto that I've seen before include "Poster (anag.)" , "Very fast (mus.)" , "To be played quickly" , "Hey . . . . ., says the magician (6)" , "Hey ..., according to the magician" .)