Stick or hold fast (6)
I believe the answer is:
adhere
I believe this is a double definition.
'stick' is the first definition.
(to adhere to something is to stick to it)
'hold fast' is the second definition.
(synonyms)
(Other definitions for adhere that I've seen before include "'Stick to, a play say (6)'" , "Header (anag)" , "Stick L" , "Stick to as by suction" , "Stick - join" .)