Cheers for model (6)
I believe the answer is:
Here is my best explanation:
'cheers' is the definition.
'for model' is the subsidiary indication.
'for' becomes 'pro' (I've seen this before).
'model' becomes 'sit' ('sit' can be a synonym of 'model').
(Other definitions for prosit that I've seen before include "I sport during German toast to your health", "Good health!; tripos (anagram)", "Good luck to you", "I sport with a drinking toast in Germany", "good health is the answer", "down the hatch", "Sir, opt for a toast", "Drinking call, good health to you", "... [good-health]" and "(German for) cheers!".)