Quickly taking break in river (6)
I believe the answer is:
presto
'quickly' is the definition.
(I've seen this before)
'break in river' is the wordplay.
'break' becomes 'rest' (a rest is taking a break).
'in' is an insertion indicator.
'river' becomes 'Po' (Italian river).
'rest' placed into '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" .)