Slander a duke during armistice (7)
I believe the answer is:
traduce
'slander' is the definition.
(traducing is a kind of slandering)
'a duke during armistice' is the wordplay.
'duke' becomes 'd'.
'during' indicates putting letters inside.
'armistice' becomes 'truce' ('truce' can be a synonym of 'armistice').
'a'+'d'='ad'
'ad' going within 'truce' is 'TRADUCE'.
(Other definitions for traduce that I've seen before include "abuse" , "Calumniate" , "Malign -- rude act (anag)" , "Defame, slander" , "Speak badly of" .)