Ask Edgar about period long ago (4,4)
I believe the answer is:
dark ages
'period long ago' is the definition.
(period of history)
'ask edgar about' is the wordplay.
'about' is an anagram indicator.
'ask'+'edgar'='askedgar'
'askedgar' anagrammed gives 'DARK AGES'.
(Other definitions for dark ages that I've seen before include "when the record is blank?" , "'Period of history between classical antiquity and the Italian Renaissance (4,4)'" , "Period in Europe approx 500 - 1100 AD" , "Period c AD500-1100" , "Period of European history from 5c to 9c or 12c" .)