Finished with American English? Too much of a good thing? (7)
I believe the answer is:
overuse
'too much of a good thing?' is the definition.
(using something too much)
'finished with american english?' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'with' says to put letters next to each other.
'american' becomes 'US' (United States).
'english?' becomes 'e' (abbreviation).
'over'+'us'+'e'='OVERUSE'
(Other definitions for overuse that I've seen before include "Excessive utilisation" , "Too much work" , "Wear out" , "Excessively utilise" , "Employ excessively" .)