Excavate part of chateau near Thionville (7)
I believe the answer is:
unearth
'excavate' is the definition.
(I've seen this before)
'part of chateau near thionville' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'UNEARTH' can be found hidden inside 'chateau near thionville'.
(Other definitions for unearth that I've seen before include "Excavate, dig up" , "Expose" , "reveal" , "Dig up for a hunter" , "Dig up - uncover" .)