Runner takes break very fast (6)
I believe the answer is:
presto
'very fast' is the definition.
(I've seen this before)
'runner takes break' is the wordplay.
'runner' becomes 'po' (Italian river).
'takes' indicates putting letters inside.
'break' becomes 'rest' (resting is a kind of breaking).
'po' enclosing 'rest' is 'PRESTO'.
(Other definitions for presto that I've seen before include "29 [ADAGIO]? On the contrary" , "hey......,cries the magician" , "at great speed..." , "Poster (anag.)" , "Hey . . . . ., says the magician (6)" .)