Number one hit about America? (6)
I believe the answer is:
nation
'america?' is the definition.
(america is a kind of nation)
'number one hit about' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'one' becomes 'i' (Roman numeral).
'hit' becomes 'tan' (tan can mean to beat or hit).
'about' is a reversal indicator.
'no'+'i'+'tan'='noitan'
'noitan' backwards is 'NATION'.
(Other definitions for nation that I've seen before include "Body of people united by their homeland" , "People of common culture or descent" , "Body of people under a single government" , "Large community of people under one government" , "The Birth of a -, D.W. Griffiths film" .)