Smart and so vain, terribly offensive (8)
I believe the answer is:
invasion
'offensive' is the definition.
(thesaurus)
'smart and so vain terribly' is the wordplay.
'smart' becomes 'in' (as in fashionable).
'and' means one lot of letters go next to another.
'terribly' indicates anagramming the letters.
'so'+'vain'='sovain'
'sovain' anagrammed gives 'vasion'.
'in'+'vasion'='INVASION'
(Other definitions for invasion that I've seen before include "Occupation" , "Unwelcome intrusion" , "Incursion, entry for conquest" , "Hostile incursion" , "Unwelcome encroachment on another country" .)