Seat almost fixed (6)
I believe the answer is:
settle
'seat' is the definition.
(settle is a kind of seat)
'almost fixed' is the wordplay.
I cannot really understand how this works, but
'fixed' could be 'set' (both can mean decided or determined) and 'set' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for settle that I've seen before include "Resolve; land" , "Come to rest or decide an argument" , "Resolve; come to rest" , "Light upon" , "Put down roots or pay debt" .)