Royal Anglo-Saxon periods (4)
I believe the answer is:
eras
'periods' is the definition.
(periods of time)
'royal anglo-saxon' is the wordplay.
'royal' becomes 'er' (abbreviation for Elizabeth Regina).
'anglo-saxon' becomes 'AS' (abbreviation for Anglo-Saxon).
'er'+'as'='ERAS'
(Other definitions for eras that I've seen before include "Epochs" , "Long and distinct periods of history" , "typeface" , "At least two times" , "Periods, ages" .)