Artist's board with thumb-hole (7)
I believe the answer is:
palette
'artist's board with thumb-hole' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for palette that I've seen before include "selection of colours" , "Artist's range of colours" , "Artist's paint mixing board" , "Variety of tones" , "Painter's board" .)