One reigning over a long period (3)
I believe the answer is:
era
'long period' is the definition.
(I've seen this before)
'one reigning over a' is the wordplay.
'one reigning' becomes 'er' (Elizabeth Regina).
'over' means one lot of letters go next to another.
'er'+'a'='ERA'
(Other definitions for era that I've seen before include "Period of alteration" , "Long historic period" , "Period, age" , "Aeon" , "generation, maybe" .)