Sort of paint that's unsuitable (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(I've seen this before)
'sort of paint' is the wordplay.
'sort of' indicates anagramming the letters.
'paint' anagrammed gives 'INAPT'.
'that's' acts as a link.
(Other definitions for inapt that I've seen before include "Inappropriate" , "Unsuitable - paint (anag)" , "Unsuited" , "unqualified" , "Useless" .)