Total agreement part of run is onerous (6)
I believe the answer is:
unison
'total agreement' is the definition.
(thesaurus)
'part of run is onerous' is the wordplay.
'part of' indicates a hidden word.
'UNISON' is hidden in the letters of 'run is onerous'.
(Other definitions for unison that I've seen before include "All voices together" , "Accord" , "All singing the same notes" , "Harmony - public services' trade union" , "using same pitch" .)