Wickedness engulfing old Swiss town (4)
I believe the answer is:
sion
'swiss town' is the definition.
(town in Switzerland)
'wickedness engulfing old' is the wordplay.
'wickedness' becomes 'sin' (synonyms).
'engulfing' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sin' enclosing 'o' is 'SION'.
(Other definitions for sion that I've seen before include "Holy place" , "Name for Jerusalem" , "Ancient Jerusalem as pictured in mansions" .)