Age even duller (4,6)
I believe the answer is:
grey matter
'age' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'even duller' is the wordplay.
'even' becomes 'grey' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'duller' becomes 'matter' ('matt' can be similar in meaning to 'dull').
'grey'+'matter'='GREY MATTER'
Can you help me to learn more?
(Other definitions for grey matter that I've seen before include "intellectual capacity?" , "stuff in the head" , "Brains" , "Intelligence; the brain" , "Brain tissue" .)