Second person from Paris informally dressed by runner for Chanel, say? (9)
I believe the answer is:
couturier
'for chanel say?' is the definition.
'second person from paris informally dressed by runner' is the wordplay.
'second person from paris informally' becomes 'tu' (informal French for 'you', grammatical meaning).
'dressed by' is an insertion indicator.
'runner' becomes 'courier' (I've seen this in another clue).
'tu' inserted into 'courier' is 'COUTURIER'.
(Other definitions for couturier that I've seen before include "Fashion designer" , "Top Gear producer" , "Designer of women's clothes" .)