The most common and most likely answer to this crossword clue is the 8 letter word ETERNITY
I've seen this crossword clue in The Times
E
T
E
R
N
I
T
Y
More clues leading to the result ETERNITY
- Tiny tree, gnarled for yonks
- Without Conservatives city centre declined seemingly forever
- Change in entirety, for all time
- Aquatic bird — it nested in banks of estuary for donkey's years
- Tiny tree struggling for a very long time
- Tiny tree flowering forever
- Teen I try to reform for a long time!
- A very long time to make a ring
- Time in entirety, perhaps
- Bird that’s coming back round initially thanks you forever
Buddy explains!

The straight part
We know that "all time" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case all time = eternity.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ETERNITY mean?
ETERNITY noun- Existence without end, infinite time.
- Existence outside of time.
- A period of time which extends infinitely far into the future.
- (metaphysical) The remainder of time that elapses after death.
- A comparatively long time.