Fairground Attraction song adopting American English (8)
I believe the answer is:
carousel
'fairground attraction' is the definition.
(type of ride at a fair)
'song adopting american english' is the wordplay.
'song' becomes 'carol' (carol is a kind of song).
'adopting' means one lot of letters goes inside another (inserted letters are taken on or adopted).
'american' becomes 'US' (United States).
'english' becomes 'E' (abbreviation).
'us'+'e'='use'
'carol' enclosing 'use' is 'CAROUSEL'.
(Other definitions for carousel that I've seen before include "Rodgers/Hammerstein hit" , "Rotating conveyor" , "fairground entertainment" , "airport feature?" , "luggage mover at Heathrow?" .)