Old Queen Anne's first important date (3)
I believe the answer is:
era
'important date' is the definition.
(I've seen this before)
'old queen anne's first' is the wordplay.
'old queen' becomes 'er' (Elizabeth Regina, ie Queen Elizabeth).
'anne's first' becomes 'a' (est letter of 'anne').
'er'+'a'='ERA'
(Other definitions for era that I've seen before include "Period of alteration" , "Major division of geological time" , "Characteristic period" , "Memorable period" , "Important time" .)