Audible river quarter (6)
I believe the answer is:
fourth
'quarter' is the definition.
(a quarter is a fourth of something)
'audible river' is the wordplay.
'audible' indicates a 'sounds like' (homophone) clue.
'river' becomes 'Forth' (Scottish river).
'forth' is a homophone of 'FOURTH'.
(Other definitions for fourth that I've seen before include "The Press is referred to as this estate" , "Just missing bronze medal?" , "Quarter" , "Behind the top three" , "(and 4) The press" .)