One reigning over a period (3)
I believe the answer is:
era
'period' is the definition.
(era is a kind of period)
'one reigning over a' is the wordplay.
'one reigning' becomes 'er' (Elizabeth Regina).
'over' is a charade indicator (letters next to each other).
'er'+'a'='ERA'
(Other definitions for era that I've seen before include "Important historical period" , "Long period of time" , "Important date" , "Cycle" , "Stage" .)