Guarantee truce is broken before start of year (8)
I believe the answer is:
security
'guarantee' is the definition.
(security is a kind of guarantee)
'truce is broken before start of year' is the wordplay.
'broken' indicates an anagram.
'before' means one lot of letters go next to another.
'start of year' becomes 'y'.
'truce'+'is'='truceis'
'truceis' with letters rearranged gives 'securit'.
'securit'+'y'='SECURITY'
(Other definitions for security that I've seen before include "Protection from possibility of future financial difficulty" , "Safety measures" , "Pledge; safety" , "Freedom from anxiety or fear" , "Protection from damage" .)