Black Country is dull (5)
I believe the answer is:
bland
'dull' is the definition.
('bland' can be similar in meaning to 'dull')
'black country' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'country' becomes 'land' ('land' can be a synonym of 'country').
'b'+'land'='BLAND'
'is' is the link.
(Other definitions for bland that I've seen before include "Lacking taste, flavour" , "Boringly characterless" , "Insipid - uninteresting" , "Suave" , "Mild and insipid" .)