Old brown exterior in a state (5)
I believe the answer is:
texan
'a state' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'old brown exterior' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'brown' becomes 'tan' (tan is a kind of brown).
'exterior' means one lot of letters goes inside another (some letters go on the outside of others).
'ex' going into 'tan' is 'TEXAN'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for texan that I've seen before include "Native of Houston, perhaps" , "One from south-west USA" , "Person from southwestern US state" , "Dallas resident?" , "From a large US state" .)