Poser demonstrating fashion line (5)
I believe the answer is:
model
'poser' is the definition.
(e.g. a model for an artist - someone who poses)
'fashion line' is the wordplay.
'fashion' becomes 'mode' (synonyms).
'line' becomes 'l' (used when specifying particular lines from a poem).
'mode'+'l'='MODEL'
'demonstrating' is the link.
(Other definitions for model that I've seen before include "Representative example" , "Artist's sitter" , "Perfect" , "who displays it" , "Ideal example" .)