Fashionable ways lead to his undoing (5)
I believe the answer is:
'fashionable' is the definition.
(I've seen this before)
'ways lead to his undoing' is the wordplay.
'ways' becomes 'sw' (I can't explain this - if you can you should believe this answer much more).
'lead' means one lot of letters go next to another.
'undoing' is an anagram indicator.
'his' is an anagram of 'ish'.
Can you help me to learn more?
(Other definitions for swish that I've seen before include "Classy" , "Elegant" , "Smart; stylish" , "Stylish (sl.)" , "Extravagant" .)