The 'pop' of 500 AD (3)
I believe the answer is:
dad
'the pop' is the definition.
(both can mean one's father)
'500 ad' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'd'+'ad'='DAD'
'of' acts as a link.
(Other definitions for dad that I've seen before include "Papa" , "Father, in short" , "Relation" , "Pa" , "Popular father - a palindrome" .)