The most common and most likely answer to this crossword clue is the 8 letter word EPIPHANY
I've seen this crossword clue in The Daily Telegraph
E
P
I
P
H
A
N
Y
More clues leading to the result EPIPHANY
- Revelation in tweet set back husband — and then some!
- Curate finally happy in restored church festival
- Hype in a rogue when seizing power produces moment of revelation
- Realisation, where I happen to change personality, ultimately
- Realisation I didn’t catch that one squeezing spot
- Husband with tube upset on some festival date
- What about offence given by some sudden revelation?
- Taking tube north, start to have some insight
- What detains Dickens hero before every realisation
- Mr Pirrip’s sorry about at least one revelation
Buddy explains!

The straight part
We know that "realisation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case realisation = epiphany.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 EPIPHANY mean?
EPIPHANY noun- The appearance of Jesus Christ to the Magi on the twelfth day after Christmas.
- An annual Christian feast celebrating this event.
- The day of the celebration, January 6th, or sometimes (in Western Christianity), the Sunday between January 2nd and 8th.
- The season or time of the Christian church year either from the Epiphany feast day to Shrove Tuesday (the day before Ash Wednesday, the start of Lent) or from the Epiphany feast day to the feast of the Presentation of Christ in the Temple.
- An illuminating realization or discovery, often resulting in a personal feeling of elation, awe, or wonder.
- A manifestation or appearance of a divine or superhuman being.