Clue: Mike and Heather enthralled by an advanced style of country music? (9)
Answer: AMERICANA
Explanation: Definition: style of country music
 [ M(letter represented by "Mike" in the phonetic alphabet) **plus**(and) ERICA(the genus of shrubs, including heather) ] **contained in**(enthralled by) [AN + A(abbrev. for "advanced") ].
Defn: …, aka American roots music, combining styles such as country and western, roots-rock, folk, bluegrass, R&B and blues.