Cuba's in genuine cessation of hostilities (5)
I believe the answer is:
truce
'cessation of hostilities' is the definition.
(I've seen this before)
'cuba's in genuine' is the wordplay.
'cuba' becomes 'c'.
'in' means one lot of letters goes inside another.
'genuine' becomes 'true' ('true' is associated in meaning with 'genuine').
'c' put within 'true' is 'TRUCE'.
(Other definitions for truce that I've seen before include "Agreement to suspend fighting" , "barley" , "peace initiative" , "Temporary ceasefire" , "halt in fighting" .)