Others entering Italian river quickly (6)
I believe the answer is:
presto
'quickly' is the definition.
(thesaurus)
'others entering italian river' is the wordplay.
'others' becomes 'rest' (I've seen this before).
'entering' is an insertion indicator.
'italian river' becomes 'po'.
'rest' put inside 'po' is 'PRESTO'.
(Other definitions for presto that I've seen before include "fast to the maximum" , "quick to score" , "Poster (anag.)" , "In quick tempo (mus.)" , "quickly in bars" .)