Unsuitable paint mixture (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(I've seen this before)
'paint mixture' is the wordplay.
'mixture' indicates anagramming the letters.
'paint' is an anagram of 'INAPT'.
(Other definitions for inapt that I've seen before include "Out of place" , "Not fitting" , "not seemly" , "Not suitable or appropriate" , "unqualified" .)