Trump's first excuse for sedition (7)
I believe the answer is:
treason
'sedition' is the definition.
(thesaurus)
'trump's first excuse' is the wordplay.
'trump's first' becomes 't' (1st letter of 'trump').
'excuse' becomes 'reason' (I've seen this before).
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "Crime of national betrayal" , "Act of betrayal of a government" , "LEse-majestE" , "Senator (anag)" , "Crime agains one's country" .)