Scrupulous but informal with thug (8)
I believe the answer is:
thorough
'scrupulous' is the definition.
(thesaurus)
'but informal with thug' is the wordplay.
'but informal' becomes 'tho' (informal spelling of 'though').
'with' says to put letters next to each other.
'thug' becomes 'rough' (both can mean a violent person).
'tho'+'rough'='THOROUGH'
(Other definitions for thorough that I've seen before include "Paying attention to detail, meticulous" , "Assiduous, painstaking" , "Attentive to detail" , "Full" , "Rigorous" .)