Quickly taking break in river (6)
I believe the answer is:
presto
'quickly' is the definition.
(I've seen this in another clue)
'break in river' is the wordplay.
'break' becomes 'rest' (a rest is taking a break).
'in' means one lot of letters goes inside another.
'river' becomes 'Po' (Italian river).
'rest' going within 'po' is 'PRESTO'.
'taking' is the link.
(Other definitions for presto that I've seen before include "To be played quickly" , "In quick tempo (mus.)" , "at great speed..." , "Hey, quickly for a magician" , "hey......,cries the magician" .)