Brainy brother drinking rye? The opposite (8)
I believe the answer is:
cerebral
'brainy' is the definition.
(I've seen this before)
'drinking rye? the opposite' is the wordplay.
'drinking' indicates putting letters inside.
'rye?' becomes 'cereal' (rye is a kind of cereal).
'the opposite' becomes 'br' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cereal' placed around 'br' is 'CEREBRAL'.
'brother' is the link.
This may not be correct. It may be part of another bit of the clue.
(Other definitions for cerebral that I've seen before include "Relating to the brain" , "Concerning the brain" , "Of the brain, intellectual" , "Intellectual, brainy" , "Highly intellectual" .)