The most common and most likely answer to this crossword clue is the 8 letter word SOMETIME
I've seen this crossword clue in The Times
S
O
M
E
T
I
M
E
More clues leading to the result SOMETIME
- Former leaders of European triumvirate inevitably going into battle
- I seem to upset when ringing married ex
- A little boy, Winchester’s sixth former
- Previous excerpt from poem, item ostensibly uplifting
- Former Tommies bursting with energy
- Well matched, I and my ex?
- Previous maths lesson read out
- Sequence of four notes from past
- 23 One day previously
- Late, therefore article returned by setter
Buddy explains!

The straight part
We know that "ex" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ex = sometime.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 SOMETIME mean?
SOMETIME adjective- Former, erstwhile; at some previous time.
- Occasional.
- At an indefinite but stated time in the past or future.
- Sometimes.
- At an unstated past or future time; once; formerly.