Facing work sitting for artist (8)
I believe the answer is:
opposing
'facing' is the definition.
(opposing is a kind of facing)
'work sitting for artist' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'sitting for artist' becomes 'posing' (I can't explain this - if you can you should give a lot more credence to this answer).
'op'+'posing'='OPPOSING'
Can you help me to learn more?
(Other definitions for opposing that I've seen before include "Going against" , "modelling after work" , "trying to end a sea of troubles by it?" , "Disapproving" , "Contending" .)