Speak badly of a duke breaking ceasefire (7)
I believe the answer is:
traduce
'speak badly of' is the definition.
(I've seen this before)
'a duke breaking ceasefire' is the wordplay.
'duke' becomes 'd' (abbreviation).
'breaking' indicates putting letters inside.
'ceasefire' becomes 'truce' (both can mean a break in fighting).
'a'+'d'='ad'
'ad' going into 'truce' is 'TRADUCE'.
(Other definitions for traduce that I've seen before include "Speak badly of" , "Tell lies about (someone)" , "'Malign, badmouth (7)'" , "Malign -- rude act (anag)" , "Speak ill of" .)