100 involved in genuine ceasefire (5)
I believe the answer is:
truce
'ceasefire' is the definition.
(both can mean a break in fighting)
'100 involved in genuine' is the wordplay.
'100' becomes 'C' (Roman numeral).
'involved in' indicates putting letters inside.
'genuine' becomes 'true' (similar in meaning).
'c' inserted inside 'true' is 'TRUCE'.
(Other definitions for truce that I've seen before include "Agreed pause in fighting" , "Respite" , "Cruet (anag.)" , "Cease-fire, armistice" , "Agreement to stop battle" .)