Back in outskirts of Derby? That’s dull (6)
I believe the answer is:
dreary
'dull' is the definition.
('dreary' can be similar in meaning to 'dull')
'back in outskirts of derby?' is the wordplay.
'back' becomes 'rear' ('rear' can be a synonym of 'back'**).
'in' indicates putting letters inside.
'outskirts of' suggests removing the centre.
'derby' with its middle taken out is 'dy'.
'rear' put into 'dy' is 'DREARY'.
'that's' acts as a link.
(Other definitions for dreary that I've seen before include "Drab and dismal" , "Gloomy, boring" , "Overcast" , "Dull and depressing" , "Dismal, depressing" .)