Defame a large number during suspension of hostilities (7)
I believe the answer is:
traduce
'defame' is the definition.
(traducing is a kind of defaming)
'a large number during suspension of hostilities' is the wordplay.
'large number' becomes 'd'.
'during' is an insertion indicator.
'suspension of hostilities' becomes 'truce' (I've seen this before).
'a'+'d'='ad'
'ad' placed into 'truce' is 'TRADUCE'.
(Other definitions for traduce that I've seen before include "Asperse" , "Speak badly of" , "Calumniate" , "Misrepresent in unfavourable way" , "Slander - misrepresent" .)