Some cube numbers can be dull (6)
I believe the answer is:
benumb
'dull' is the definition.
('benumb' can be a synonym of 'dull')
'some cube numbers can' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'BENUMB' is hidden within 'cube numbers can'.
'be' is the link.
(Other definitions for benumb that I've seen before include "Stun" , "Maybe anaesthetise" , "Deprive of feeling" , "Make dazed or unfeeling" .)