A nut is loose in Texas (6)
I believe the answer is:
austin
'in texas' is the definition.
('Austin' is part of 'texas')
'a nut is loose' is the wordplay.
'loose' indicates anagramming the letters (letters are loosened from their original positions).
'nut'+'is'='nutis'
'nutis' anagrammed gives 'ustin'.
'a'+'ustin'='AUSTIN'
(Other definitions for austin that I've seen before include "The capital of Texas (6)" , "Engineer" , "Texan city" , "Capital city of Texas" , "US state capital - old English car manufacturer" .)