Pressure beginning to weaken cube (6)
I believe the answer is:
weight
'pressure' is the definition.
(I've seen this before)
'beginning to weaken cube' is the wordplay.
'beginning to' indicates taking the first letters.
'cube' becomes 'eight' (8 is a cube number).
The initial letter of 'weaken' is 'w'.
'w'+'eight'='WEIGHT'
(Other definitions for weight that I've seen before include "Importance, oppressive force" , "Trouble" , "Significance" , "gives a sense of oppression" , "Gravitational pull" .)