Underwear thief in Kansas (8)
I believe the answer is:
knickers
'underwear' is the definition.
(I've seen this before)
'thief in kansas' is the wordplay.
'thief' becomes 'nicker' (I've seen this before).
'in' is an insertion indicator.
'kansas' becomes 'ks'.
'nicker' put within 'ks' is 'KNICKERS'.
(Other definitions for knickers that I've seen before include "Garment (not to be twisted!)" , "Mild expletive" , "Pants" , "Risk neck (anag.)" , "Underwear - oh dear!" .)