Run into a fashionable dog (5)
I believe the answer is:
incur
'run into' is the definition.
(I've seen this in another clue)
'a fashionable dog' is the wordplay.
'a fashionable' becomes 'in' (in can mean fashionable or trendy).
'dog' becomes 'cur' (both can mean a despicable person).
'in'+'cur'='INCUR'
(Other definitions for incur that I've seen before include "Take on (obligation)" , "Become subject to (something unpleasant)" , "Become subject to (eg, taxes)" , "Arouse" , "Run up" .)