Mike and Heather enthralled by an advanced style of country music? (9)
I believe the answer is:
'style of country music?' is the definition.
(genre of music based on American traditions)
'mike and heather enthralled by an advanced' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' says to put letters next to each other.
'heather' becomes 'erica' (the plant genus Erica includes heather plants).
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'advanced' becomes 'a' (abbreviation).
'merica' put inside 'an' is 'american'.
(Other definitions for americana that I've seen before include "Things from the USA" , "Yank stuff" , "Things associated with the US" , "Objects from the New World" , "Wild West material?" .)