All the universe except the speed of light is an assimilation process (7)
I believe the answer is:
osmosis
'an assimilation process' is the definition.
(osmosis can mean a process of assimilation)
'all the universe except the speed of light is' is the wordplay.
'all the universe' becomes 'cosmos' (the cosmos is the universe).
'except' is a deletion indicator.
'the speed of light' becomes 'c' (as in E=mc2).
'cosmos' with 'c' removed is 'osmos'.
'osmos'+'is'='OSMOSIS'
(Other definitions for osmosis that I've seen before include "Something like dialysis" , "Diffusion of liquid through porous barrier" , "Process of gradual absorption" , "Process of learning something gradually and without effort" , "Diffusion of liquid through porous membrane" .)