Conservative breaks genuine peaceful agreement (5)
I believe the answer is:
truce
'peaceful agreement' is the definition.
(peace agreed after a war)
'conservative breaks genuine' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'breaks' indicates putting letters inside.
'genuine' becomes 'true' (similar in meaning).
'c' going within 'true' is 'TRUCE'.
(Other definitions for truce that I've seen before include "Suspension of war" , "Temporary halt in hostilities" , "Agreement to stop fighting temporarily" , "Respite" , "Agreed pause in fighting" .)