At no time in the past or future (5)
I believe the answer is:
never
'at no time in the past or future' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for never that I've seen before include "when pigs fly" , "Emphatic negative" , "At no time in the past or future" , "when hell freezes over?" , "Strong negative" .)