19 down
Frost
ultimately
breaks
hole
in
crumbling
relic
(7)
Frost ultimately breaks hole in crumbling relic (7)
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The definition is at the end of the clue.
How the clue works
The definition is relic, referring to a Neolithic stone-age artefact. Taking T (the ultimate letter of "Frost") together with the letters of "hole in" and anagramming the whole lot (indicated by "crumbling") gives NEOLITH.