Sometime in the Holocene Epoch? (3,2,5,4)
I believe the answer is:
one of these days
'sometime' is the definition.
Both the answer and definition are adverbs. Perhaps you can see an association between them that I don't see?
'the holocene epoch?' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'epoch?' could be 'day' (day is a kind of epoch) and 'day' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for one of these days that I've seen before include "Some time soon" , "At some time in the near future" , "Sooner or later" , "provide a selection for a date in the diary?" .)