Frank is dull (5)
I believe the answer is:
blunt
I believe this clue is a double definition.
'frank' is the first definition.
('blunt' can be a synonym of 'frank')
'dull' is the second definition.
('blunt' can be a synonym of 'dull')
'is' is the link.
(Other definitions for blunt that I've seen before include "To the point; without point" , "Unsharpened" , "Frank and forthright" , "Straightforward" , "Take the edge off" .)