Part of painter's work starts off collection of art treasures (4)
I believe the answer is:
coat
'part of painter's work' is the definition.
(as in a coat of paint)
'starts off collection of art treasures' is the wordplay.
'starts off' indicates taking the first letters.
The initial letters of 'collection of art treasures' is 'COAT'.
(Other definitions for coat that I've seen before include "Outer covering" , "blanket" , "Warm outerwear" , "Layer of paint" , "Fleece" .)