The purpose of American English? (3)
I believe the answer is:
use
'the purpose' is the definition.
('use' can be a synonym of 'purpose')
'american english?' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'english?' becomes 'e' (abbreviation).
'us'+'e'='USE'
'of' acts as a link.
(Other definitions for use that I've seen before include "Benefit" , "Manipulate" , "Take drugs regularly" , "Resort to - operation" , "Apply, employ" .)