Stick together or split apart (6)
I believe the answer is:
cleave
This clue is a double definition.
'stick together' is the first definition.
(thesaurus)
'split apart' is the second definition.
(I've seen this in another clue)
'or' acts as a link.
(Other definitions for cleave that I've seen before include "remain attached" , "Cut (with an axe?)" , "Split as with axe" , "Split with the grain" , "Stick fast; split" .)