Intelligence is a boring topic (4,6)
I believe the answer is:
grey matter
'intelligence' is the definition.
(I've seen this before)
'boring topic' is the wordplay.
'boring' becomes 'grey' (I can't justify this - if you can you should believe this answer much more).
'topic' becomes 'matter' (synonyms).
'grey'+'matter'='GREY MATTER'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for grey matter that I've seen before include "Brains" , "Intelligence; the brain" , "intellectual capacity?" , "Brain tissue" , "Issue lacking clear boundaries" .)