Offensive verse entering fashionable area, a Swiss town (8)
I believe the answer is:
invasion
'offensive' is the definition.
(I've seen this before)
'verse entering fashionable area a swiss town' is the wordplay.
'verse' becomes 'v'.
'entering' means one lot of letters goes inside another.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'area' becomes 'a' (maths abbreviation).
'a swiss town' becomes 'sion' (I've seen this in another clue).
'in'+'a'='ina'
'v' inserted inside 'ina' is 'inva'.
'inva'+'sion'='INVASION'
(Other definitions for invasion that I've seen before include "Unwelcome encroachment on another country" , "Impetuous; skin eruption" , "Incursion, entry for conquest" , "Encroachment by enemy" , "Incursive attack" .)