The French pen started fashion (5)
I believe the answer is:
style
'fashion' is the definition.
(I know that fashion can be written as style)
'the french pen started' is the wordplay.
'the french' becomes 'le' ('the' in French).
'pen' becomes 'sty' (both can mean an animal enclosure).
'started' means one lot of letters go next to another.
'le' put after 'sty' is 'STYLE'.
(Other definitions for style that I've seen before include "Manner or way of doing something" , "Superior quality" , "Elan" , "With 1a [NEW] dating from 1752" , "Distinctive manner" .)