Head turned by US neighbour's cheerfulness (8)
I believe the answer is:
bonhomie
'cheerfulness' is the definition.
'head turned by us neighbour's' is the wordplay.
'head' becomes 'nob' (I've seen this in other clues).
'turned' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'us neighbour' becomes 'homie'.
'nob' in reverse letter order is 'bon'.
'bon'+'homie'='BONHOMIE'
(Other definitions for bonhomie that I've seen before include "good humour" , "Exuberant friendliness - goodwill" , "'Affability, geniality (8)'" , "Genial manner from hobo in me" , "Warmth" .)