Intelligence? Dull subject! (4,6)

I believe the answer is:

grey matter

'intelligence?' is the definition.
(I've seen this before)

'dull subject' is the wordplay.
'dull' becomes 'grey' (I've seen this before).
'subject' becomes 'matter' ('matter' can be a synonym of 'subject'**).
'grey'+'matter'='GREY MATTER'

(Other definitions for grey matter that I've seen before include "Brain tissue" , "Issue lacking clear boundaries" , "intellectual capacity?" , "Intelligence; the brain" , "stuff in the head" .)

I've seen this clue in The Telegraph.

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