The most common and most likely answer to this crossword clue is the 5 letter word TRULY
I've seen this crossword clue in The Guardian
T
R
U
L
Y
Buddy explains!

The straight part
We know that "Really" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case really = truly.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 TRULY mean?
TRULY adverb- (manner) In accordance with the facts; truthfully, accurately.
- (modal) Honestly, genuinely, in fact, really.
- (degree) Very.