Grain entertaining British intellectual (8)
I believe the answer is:
cerebral
'intellectual' is the definition.
(I know that intellectual can be written as cerebral)
'grain entertaining british' is the wordplay.
'grain' becomes 'cereal' (synonyms).
'entertaining' means one lot of letters goes inside another (inviting in, as for dinner).
'british' becomes 'br' (abbreviation for British).
'cereal' placed around 'br' is 'CEREBRAL'.
(Other definitions for cerebral that I've seen before include "Of the brain, intellectual" , "Concerning the brain" , "Intellectual, brainy" , "Relating to the brain" , "Highly intellectual" .)