Too American -- 'Dandy!'
I believe the answer is:
as well
'too' is the definition.
('as well' can be a synonym of 'too')
'american dandy' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'dandy' becomes 'swell' ('swell' can be a synonym of 'dandy').
'a'+'swell'='AS WELL'
(Other definitions for as well that I've seen before include "with equal effect" , "A swell sort of addition" , "To boot" , "Also, in addition" , "Appropriate" .)