Thieves after a thousand items of underwear (8)
I believe the answer is:
knickers
'items of underwear' is the definition.
'thieves after a thousand' is the wordplay.
'thieves' becomes 'nickers' (nick can mean to steal**).
'after' is a charade indicator (letters next to each other).
'a thousand' becomes 'k' ('K' can be a synonym of 'thousand').
'nickers' after 'k' is 'KNICKERS'.
(Other definitions for knickers that I've seen before include "Underwear - oh dear!" , "Undergarments for women" , "Garment (not to be twisted!)" , "Pants" , "Panties" .)