Expedition missing out top of Umbrian peak (3)

I believe the answer is:

tor

'peak' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for tor that I've seen before include "Rot about the rocky height" , "Rocky outcrop" , "Rising" , "High, rocky, bare hill" , "Fermenting agent" .)

I've seen this clue in The Times.

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