Frank is not attractive (5)
I believe the answer is:
plain
I believe this clue is a double definition.
'frank' is the first definition.
(I've seen this before)
'not attractive' is the second definition.
(I've seen this before)
'is' acts as a link.
(Other definitions for plain that I've seen before include "Simple, unadorned" , "Simple or apparent" , "Ordinary; area of land" , "Unvarnished" , "not attractive" .)