Disapprove of operation on model (6)
I believe the answer is:
oppose
'disapprove of' is the definition.
(thesaurus)
'operation on model' is the wordplay.
'operation' becomes 'op'.
'on' is a charade indicator (letters next to each other).
'model' becomes 'pose' (synonyms).
'op'+'pose'='OPPOSE'
(Other definitions for oppose that I've seen before include "Refute - confront" , "Take stand against, resist" , "Bar" , "Resist; disapprove of" , "Resist or fight against" .)