One of these indicates indefinite futurity (4)
I believe the answer is:
days
'indicates indefinite futurity' is the definition.
I can't judge whether this defines the answer.
'one of these' is the wordplay.
I cannot really understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
The remaining letters 'dys' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for days that I've seen before include "Twenty-four hour periods" , "Periods of 24 hours" , "Sun, Mon, Tues, ..." , "Units of time" , "Periods (of week)" .)