Fastener twisted round tree — a point one observes (10)
I believe the answer is:
eyewitness
'fastener' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'twisted round tree a point one observes' is the wordplay.
I cannot quite see how this works, but
'tree' could be 'yew' (type of tree) and 'yew' is located in the answer.
'point' could be 'ne' (NE is an example) and 'ne' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found 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 eyewitness that I've seen before include "Spectator" , "Onlooker" , "One who sees what happens" , "One seeing (eg, crime)" , "The Observer" .)