Assumed names as found in American stories (7)
I believe the answer is:
aliases
'assumed names' is the definition.
('alias' can be a synonym of 'assumed name')
'as found in american stories' is the wordplay.
'found in' is an insertion indicator.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'stories' becomes 'lies' (to tell stories can mean to lie).
'a'+'lies'='alies'
'as' inserted into 'alies' is 'ALIASES'.
(Other definitions for aliases that I've seen before include "Assumed names" , "handles affected?" , "False identities" , "Assumed identities" , "A lassie with false names" .)