Disloyal peacekeepers leading to ceasefire killing hundred (6)
I believe the answer is:
untrue
'disloyal' is the definition.
(similar in meaning)
'peacekeepers leading to ceasefire killing hundred' is the wordplay.
'peacekeepers' becomes 'UN' (United Nations, peacekeeping organisation).
'leading to' means one lot of letters go next to another (some letters go before or lead others).
'ceasefire' becomes 'truce' (I've seen this before).
'killing' suggests deleting specific letters.
'hundred' becomes 'c' (Roman numeral).
'truce' with 'c' taken away is 'true'.
'un'+'true'='UNTRUE'
(Other definitions for untrue that I've seen before include "Not loyal" , "Incorrect or false" , "Not in accordance with the facts" , "False or crooked" , "disloyal" .)