Many involved in genuine cease-fire (5)
I believe the answer is:
truce
'cease-fire' is the definition.
('truce' can be a synonym of 'cease-fire')
'many involved in genuine' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'involved in' means one lot of letters goes inside another.
'genuine' becomes 'true' ('true' is associated in meaning with 'genuine').
'c' going inside 'true' is 'TRUCE'.
(Other definitions for truce that I've seen before include "Temporary halt in hostilities" , "Agreed pause in fighting" , "Suspension of hostilities" , "Suspension of war" , "barley" .)