To separate and to adhere (6)
I believe the answer is:
cleave
'to separate and to adhere' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cleave that I've seen before include "Split as with axe" , "Adhere or split apart" , "Open" , "Adhere - or chop as with axe" , "Split with the grain" .)