River stopping quiet monster's advance (8)
I believe the answer is:
progress
'advance' is the definition.
(to progress is to advance)
'river stopping quiet monster's' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'monster' becomes 'ogress' (female ogre).
'p'+'ogress'='pogress'
'r' going into 'pogress' is 'PROGRESS'.
(Other definitions for progress that I've seen before include "Get on with" , "Breakthrough - advance" , "Move forward" , "Mature" , "Advancement" .)