Manage to get firmly fixed (6)
I believe the answer is:
secure
'fixed' is the definition.
(thesaurus)
'manage to get firmly' is the wordplay.
I cannot really understand how this works, but
'to get' could be 're' (I've seen this in other clues) and 're' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for secure that I've seen before include "Safe and unthreatened" , "Fix; safe" , "in the bag" , "Firm; safe" , "'Safe, confident (6)'" .)