I'm acting as a model, impressive in appearance (8)
I believe the answer is:
imposing
'impressive in appearance' is the definition.
(I've seen this before)
'i'm acting as a model' is the wordplay.
'acting as a model' becomes 'posing' (I've seen this before).
'im'+'posing'='IMPOSING'
(Other definitions for imposing that I've seen before include "Grand, impressive" , "Taking liberties" , "Enormous - levying - striking" , "Impressive - or putting tax on" , "Impressive in appearance" .)