Very concerning to come across a betrayal (7)
I believe the answer is:
treason
'betrayal' is the definition.
('treason' can be a synonym of 'betrayal')
'very concerning to come across a' is the wordplay.
'very' becomes 'tres' (French).
'concerning' becomes 'on'.
'to come across' means one lot of letters goes inside another.
'tres'+'on'='treson'
'treson' placed around 'a' is 'TREASON'.
(Other definitions for treason that I've seen before include "Mutiny" , "Offence against state" , "Criminal attempt to overthrow a government" , "Betrayal of the state" , "Crime of national betrayal" .)