Can possibly take three-quarters of an hour to fix firmly (6)
I believe the answer is:
anchor
'fix firmly' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for anchor that I've seen before include "Device used to prevent vessel from drifting" , "Ship's mooring at sea" , "Last runner in relay race team" , "Horse-breeding farm" , "ground" .)