Exercise in American English (3)
I believe the answer is:
use
'exercise' is the definition.
(I know that exercise is a more specific form of the action the use)
'american english' is the wordplay.
'american' becomes 'US' (United States).
'english' becomes 'E' (abbreviation as in OED).
'us'+'e'='USE'
'in' acts as a link.
(Other definitions for use that I've seen before include "Resort to - operation" , "Employ or exploit" , "Employment; utilise" , "Put into service, do something with" , "Put to work" .)