Rival ducks maintain very quiet whistle (8)
I believe the answer is:
opposing
'rival' is the definition.
(thesaurus)
'ducks maintain very quiet whistle' is the wordplay.
'ducks' becomes 'oo' (I've seen this in other clues).
'maintain' is an insertion indicator.
'very quiet' becomes 'pp'.
'whistle' becomes 'sing' ('sing' can be a synonym of 'whistle').
'oo' enclosing 'pp' is 'oppo'.
'oppo'+'sing'='OPPOSING'
(Other definitions for opposing that I've seen before include "Contending" , "Going against" , "modelling after work" , "Disapproving" , "trying to end a sea of troubles by it?" .)