American uncle plays a major part (3)
I believe the answer is:
sam
'american uncle' is the definition.
(Uncle Sam is a nickname for the USA)
'plays a major part' is the wordplay.
'part' indicates a hidden word.
'SAM' is hidden in the letters of 'plays a major'.
(Other definitions for sam that I've seen before include "together once" , "Boy" , "Uncle" , "Casablanca piano player" .)