Concept of immeasurable time, in its entirety, perplexes them (8)
I believe the answer is:
eternity
'concept of immeasurable time' is the definition.
'eternity' can be an answer for 'time' (eternity is a kind of time). I am unsure of the 'concept of immeasurable' bit.
'its entirety perplexes them' is the wordplay.
'entirety' anagrams to 'ETERNITY'.
But, I am not clear how this is indicated.
'in' is the link.
Can you help me to learn more?
(Other definitions for eternity that I've seen before include "Time without end" , "Ages and ages" , "Perpetuity" , "Endless period" , "Wolfgang Amadeus -" .)