Intellectual brother tucked into breakfast food (8)
I believe the answer is:
cerebral
'intellectual' is the definition.
('cerebral' can be a synonym of 'intellectual')
'brother tucked into breakfast food' is the wordplay.
'brother' becomes 'br'.
'tucked into' means one lot of letters goes inside another.
'breakfast food' becomes 'cereal' (cereal is a kind of breakfast food).
'br' placed inside 'cereal' is 'CEREBRAL'.
(Other definitions for cerebral that I've seen before include "Concerning the brain" , "Relating to the brain" , "Intellectual, brainy" , "Of the brain, intellectual" , "Highly intellectual" .)