Arrest adult trio that's regularly expressed upsetting language (7)
I believe the answer is:
italian
'language' is the definition.
(Italian is a language)
'arrest adult trio that's regularly expressed upsetting' is the wordplay.
'arrest' becomes 'nail' (synonyms).
'adult' becomes 'a' (abbreviation).
'that's regularly expressed' means one should take alternating letters (letters taken at regular intervals).
'upsetting' is a reversal indicator.
The alternate letters of 'trio' are 'ti'.
'nail'+'a'+'ti'='nailati'
'nailati' reversed gives 'ITALIAN'.
(Other definitions for italian that I've seen before include "From Rome, for example" , "Relating to a Neapolitan, say" , "Nationality of benito or Giuseppe" , "European language" , "From Naples or Turin, perhaps?" .)