There's many an advert for pop (3)
I believe the answer is:
dad
'for pop' is the definition.
(both can mean one's father)
'there's many an advert' is the wordplay.
'there's' means one lot of letters go next to another.
'many' becomes 'd' (Roman numeral for 500).
'an advert' becomes 'ad' ('ad' can be a synonym of 'advert').
'd'+'ad' is 'DAD'.
(Other definitions for dad that I've seen before include "Pa" , "Popular father - a palindrome" , "Papa" , "Relation" , "Popular and informal father" .)