Working with trustee in secret dispute (10)
I believe the answer is:
controvert
'dispute' is the definition.
(thesaurus)
'working with trustee in secret' is the wordplay.
'working' becomes 'on'.
'with' is a charade indicator (letters next to each other).
'trustee' becomes 'tr' (I've seen this in other clues).
'in' indicates putting letters inside.
'secret' becomes 'covert' (similar in meaning).
'on'+'tr'='ontr'
'ontr' put within 'covert' is 'CONTROVERT'.
(Other definitions for controvert that I've seen before include "Argue against" , "Wrangle" .)