A hundred break truce over New Year (7)
I believe the answer is:
century
'a hundred' is the definition.
(synonyms)
'break truce over new year' is the wordplay.
'break' is an anagram indicator.
'over' is an insertion indicator.
'new' becomes 'n' (abbreviation as in New Testament NT).
'year' becomes 'y'.
'truce' with letters rearranged gives 'cetur'.
'cetur' going around 'n' is 'centur'.
'centur'+'y'='CENTURY'
(Other definitions for century that I've seen before include "Period of time" , "A lot of runs!" , "Group of a hundred" , "100 years" , "One hundred years" .)