Model intoxicated, so stay put (3,5)
I believe the answer is:
sit tight
'stay put' is the definition.
(I've seen this in another clue)
'model intoxicated' is the wordplay.
'model' becomes 'sit' (e.g. for a painting).
'intoxicated' becomes 'tight' (similar in meaning).
'sit'+'tight'='SIT TIGHT'
'so' is the link.
(Other definitions for sit tight that I've seen before include "Be too drunk to stand?" , "Keep one's position" , "Maintain one's seat" , "Remain in sedentary posture, take no action" , "Remain firmly in one's place" .)