Back in Derby on vacation – that's dull (6)
I believe the answer is:
dreary
'dull' is the definition.
(similar in meaning)
'back in derby on vacation' is the wordplay.
'back' becomes 'rear' (synonyms).
'in' indicates putting letters inside.
'on vacation' says to hollow out the word (remove centre letters).
'derby' with its middle removed is 'dy'.
'rear' placed inside 'dy' is 'DREARY'.
'that's' acts as a link.
(Other definitions for dreary that I've seen before include "Dull, tedious" , "Overcast" , "Dismal, funeral" , "Grey and dull" , "Gloomy - dull" .)