Mean shortly to enclose grey brain part (7)
I believe the answer is:
medulla
'brain part' is the definition.
('medulla' is part of 'brain')
'mean shortly to enclose grey' is the wordplay.
'shortly' means to remove the last letter.
'to enclose' means one lot of letters goes inside another.
'grey' becomes 'dull' (I can't justify this - if you can you should believe this answer much more).
'mean' with its final letter taken off is 'mea'.
'mea' enclosing 'dull' is 'MEDULLA'.
Can you help me to learn more?
(Other definitions for medulla that I've seen before include "Innards of organ" , "centre of organ" , "pith" , "Lower part of the brain" , "bit of brain" .)