Stick fast (6)
I believe the answer is:
adhere
'stick fast' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for adhere that I've seen before include "'Stick to, a play say (6)'" , "Stick to as by suction" , "Stick fast (to something)" , "Keep" , "Stick to or continue faithful" .)