Extremely wide cube is a burden (4,6)
I believe the answer is:
dead weight
'a burden' is the definition.
(I've seen this before)
'extremely wide cube' is the wordplay.
'extremely' becomes 'dead' (as in a phrase like 'dead easy').
'wide' becomes 'w' (abbreviation e.g. in cricket).
'cube' becomes 'eight' (8 is a cube number).
'dead'+'w'+'eight'='DEAD-WEIGHT'
'is' acts as a link.
(Other definitions for dead weight that I've seen before include "burden" , "Ship's heaviest load" , "An encumbrance" , "Total load on ship" , "Total cargo a ship can carry" .)