Self-contained area that's relatively dull? (6,4)
I believe the answer is:
granny flat
'self-contained area' is the definition.
The definition suggests a singular noun which matches the answer.
'relatively dull?' is the wordplay.
'relatively' becomes 'granny' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dull?' becomes 'flat' ('flat' can be similar in meaning to 'dull').
'granny'+'flat'='GRANNY FLAT'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for granny flat that I've seen before include "Accommodation for old" , "separate quarters" , "section of house?" .)