Dull person getting a couple of books in prison (7)
I believe the answer is:
cabbage
'dull person' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'a couple of books in prison' is the wordplay.
'couple of' shows that letters should be duplicated.
'books' becomes 'b' (this might be a standard abbreviation I've not previously seen).
'in' indicates putting letters inside.
'prison' becomes 'cage' (cage is a kind of prison).
'b' doubled is 'bb'.
'a'+'bb'='abb'
'abb' put inside 'cage' is 'CABBAGE'.
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for cabbage that I've seen before include "Veg" , "Savoy vegetable" , "Leafy vegetable" , "Beg a cab to take the vegetable" , "Kind of 13" .)