Time out of time — it's simple (4)
I believe the answer is:
mere
This could be a double definition.
'time out of time' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'simple' is the second definition.
(synonyms)
'it's' acts as a link.
(Other definitions for mere that I've seen before include "Lake in Cumbria" , "President Hollande's mother?" , "of no significance" , "Lake, pond (poetic)" , "Nothing more than what is specified (4)" .)