School uniform matches period in history (9)
I believe the answer is:
seventies
'period in history' is the definition.
(a decade of history)
'school uniform matches' is the wordplay.
'school' becomes 'S'.
'uniform' becomes 'even' (both can mean equal or level).
'matches' becomes 'ties' (tie can mean a sporting fixture).
's'+'even'+'ties'='SEVENTIES'
(Other definitions for seventies that I've seen before include "retirement years?" , "Decade of the Arms crisis, Vietnam and All Kinds of Everything" .)