Dull friend who shares accommodation (8)
I believe the answer is:
flatmate
'shares accommodation' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'dull friend' is the wordplay.
'dull' becomes 'flat' (synonyms).
'friend' becomes 'mate' (mate is a kind of friend).
'flat'+'mate'='FLATMATE'
'who' acts as a link.
Can you help me to learn more?
(Other definitions for flatmate that I've seen before include "Sharer of accommodation" , "Cohabitee" , "for Holmes: Watson?" , "fellow lodger" , "Fellow apartment resident" .)