... one concealed beneath Tolkien creature (6)
I believe the answer is:
orchid
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'concealed beneath tolkien creature' is the wordplay.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'beneath' says to put letters next to each other (in a down clue).
'tolkien creature' becomes 'orc' (orcs are creatures in The Lord of the Rings).
'hid' after 'orc' is 'ORCHID'.
Can you help me to learn more?
(Other definitions for orchid that I've seen before include "Exotic flower with strong perfume" , "Beautiful exotic flower" , "Often prized flowering plant" , "Plant with complex blooms" , "Wild (or much cultivated) flower" .)