Stick fast to (6)
'stick fast to' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for adhere that I've seen before include "Stay attached" , "Hold fast, stand by" , "Header (anag)" , "Stick to, remain loyal" , "Agree" .)
(Other definitions for cleave that I've seen before include "Divide by chopping, or cling" , "Split with the grain" , "Stick fast; split" , "Part" , "Cut (with an axe?)" .)