Unbackable interment in flash tombstone (8)
I believe the answer is:
monument
'tombstone' is the definition.
(thesaurus)
'unbackable interment in flash' is the wordplay.
I cannot quite see how this works, but
'flash' could be 'mo' (both mean a very short period of time) and 'mo' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for monument that I've seen before include "Lasting record" , "London tube station" , "Commemorative structure" , "Commemorative column etc" , "Memorial structure" .)