So tube can be dull (6)
I believe the answer is:
obtuse
'dull' is the definition.
(I know that dull can be written as obtuse)
'so tube can be' is the wordplay.
'can be' indicates an anagram.
'so'+'tube'='sotube'
'sotube' is an anagram of 'OBTUSE'.
(Other definitions for obtuse that I've seen before include "Blunt, not sensitive" , "Exaggerate" , "Angle greater than 90 degrees" , "Slow of understanding" , "boneheaded?" .)