Go up to extremely rare mythical creature (4)
I believe the answer is:
ogre
'mythical creature' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'go up to extremely rare' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'to extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'rare' with its centre taken out is 're'.
'go' back-to-front is 'og'.
'og'+'re'='OGRE'
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "Grim(m) beast?" , "Fairy tale monster" , "terrible person" , "Monstrous creature" , "Wicked and inhuman person" .)