Lo, an egg may be a thousand years old! (3-4)
I believe the answer is:
age-long
'a thousand years old' is the definition.
The definition suggests an adjective which matches the answer.
'lo an egg may be' is the wordplay.
'may be' indicates anagramming the letters.
'lo'+'an'+'egg'='loanegg'
'loanegg' anagrammed gives 'AGE-LONG'.
Can you help me to learn more?
(Other definitions for age-long that I've seen before include "Seeming to have always been so" , "Having existed for many a year" .)