Gradual assimilation of silicon in universe, not carbon (7)
I believe the answer is:
osmosis
'gradual assimilation' is the definition.
(I've seen this before)
'silicon in universe not carbon' is the wordplay.
'silicon' becomes 'si' (Si is the chemical symbol for silicon).
'in' is an insertion indicator.
'universe' becomes 'cosmos' ('cosmos' can be a synonym of 'universe').
'not' suggests deleting specific letters.
'carbon' becomes 'c' (C is the chemical symbol for carbon).
'si' inserted inside 'cosmos' is 'cosmosis'.
'cosmosis' with 'c' taken out is 'OSMOSIS'.
'of' is the link.
(Other definitions for osmosis that I've seen before include "Diffusion through a semi-permeable membrane" , "Passage of bodily fluid through porous partition" , "Becoming absorbed" , "Diffusion; assimilation" , "Process of unconscious assimilation" .)