Agree to a rise, one hears (6)
I believe the answer is:
assent
'agree to' is the definition.
(I've seen this before)
'a rise one hears' is the wordplay.
'a rise' becomes 'ascent' ('ascent' can be a synonym of 'rise').
'one hears' shows a homophone (sound like) (I've seen 'hear' mean this).
'ascent' is a homophone of 'ASSENT'.
(Other definitions for assent that I've seen before include "Acquiescence" , "Accede, acquiesce" , "Nod" , "Official sanction" , "Express approval or agreement" .)