Firmly fixed or free from uncertainty (6)
I believe the answer is:
secure
'firmly fixed or free from uncertainty' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for secure that I've seen before include "Seize; make fast" , "Free from danger and financially sound" , "Free from danger or risk" , "properly restrained" , "Safe against attack or well fastened" .)