Fasten belt finally covered by some fullness (7)
I believe the answer is:
satiety
'some fullness' is the definition.
(satiety is a kind of fullness)
'fasten belt finally covered by' is the wordplay.
I cannot quite understand how this works, but
'fasten' could be 'tie' (tying is a kind of fastening) and 'tie' is present in the answer.
'by' could be 'at' and 'at' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for satiety that I've seen before include "Having enough" , "Full state" , "Repletion" , "Feeling of fullness" , "Boredom" .)