Model wearing a medical dressing (7)
I believe the answer is:
plaster
'medical dressing' is the definition.
(I've seen this before)
'model wearing a' is the wordplay.
'model' becomes 'last' (model used by a shoemaker).
'wearing' is an insertion indicator.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'last' inserted inside 'per' is 'PLASTER'.
(Other definitions for plaster that I've seen before include "____ of Paris" , "Wall surface" , "Small wound covering" , "Strip of material for dressing wound" , "Adhesive material" .)