Head must accept restriction in range of colours (7)
I believe the answer is:
palette
'range of colours' is the definition.
(I've seen this before)
'head must accept restriction' is the wordplay.
'head' becomes 'pate' (I've seen this before).
'must accept' means one lot of letters goes inside another.
'restriction' becomes 'let' (I am not sure about this - if you are sure you should believe this answer much more).
'pate' enclosing 'let' is 'PALETTE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for palette that I've seen before include "Artist's board with thumb-hole" , "An artist mixes his paints on it" , "Paint-mixing board" , "Board for an artist" , "Painter's utensil" .)