Brain tissue; intelligence (4,6)
I believe the answer is:
grey matter
This clue is a double definition.
'brain tissue' is the first definition.
(I've seen this before)
'intelligence' is the second definition.
(I've seen this before)
(Other definitions for grey matter that I've seen before include "Brains" , "intellectual capacity?" , "Intelligence; the brain" , "Issue lacking clear boundaries" , "Brain tissue" .)