Unsuitable replacement for paint (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(I've seen this before)
'replacement for paint' is the wordplay.
'replacement for' indicates an anagram.
'paint' with letters rearranged gives 'INAPT'.
(Other definitions for inapt that I've seen before include "Unsuitable - paint (anag)" , "Far from fit" , "Improper" , "Not fit, unsuitable" , "Not suitable or appropriate" .)