Mixed paint is not suitable (5)
I believe the answer is:
inapt
'not suitable' is the definition.
(I've seen this before)
'mixed paint' is the wordplay.
'mixed' is an anagram indicator.
'paint' with letters rearranged gives 'INAPT'.
'is' acts as a link.
(Other definitions for inapt that I've seen before include "Inappropriate" , "badly judged" , "Free from deceit" , "Not fitting" , "Not proper" .)