In a certain state (5)
I believe the answer is:
texan
'certain state' is the definition.
Although both the answer and definition are adjectives, I don't see how one could define the other.
'in a' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found in the answer.
The remaining letters 'tex' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for texan that I've seen before include "Dallas resident?" , "One from SW USA" , "One from south-west USA" , "Native of Houston, perhaps" , "From a large US state" .)