Mark's trendy way of speaking including American (10)
I believe the answer is:
indication
'mark's' is the definition.
(thesaurus)
'trendy way of speaking including american' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'way of speaking' becomes 'diction' (I've seen this before).
'including' indicates putting letters inside.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'diction' going around 'a' is 'dication'.
'in'+'dication'='INDICATION'
(Other definitions for indication that I've seen before include "Sign (of something)" , "it shows" , "form of evidence" .)