Part of brain that mostly inverted everything? Nearly everything (8)
I believe the answer is:
thalamus
'part of brain' is the definition.
(I know that thalamus is part of diencephalon)
'that mostly inverted everything? nearly everything' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'inverted' says the letters should be written backwards.
'everything?' becomes 'sum' (I've seen this in another clue).
'nearly' means to remove the last letter.
'everything' becomes 'all' (all things).
'all' with its final letter removed is 'al'.
'sum'+'al'='sumal'
'sumal' back-to-front is 'lamus'.
'that' with its final letter removed is 'tha'.
'tha'+'lamus'='THALAMUS'
(Other definitions for thalamus that I've seen before include "a bit of brains" , "Sensory part of brain; a lush mat (anag.)" , "bit of brain" , "Grey matter at base of brain" .)