Highest nest is one of these things (7)
I believe the answer is:
anagram
'highest nest' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one of these things' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
(Other definitions for anagram that I've seen before include "Eg "oscillation" of "colonialist"" , "Argue the point" , "Word arranging the letters of another" , "Letter puzzle" , "Word(s) rearranged to form others" .)