The most common and most likely answer to this crossword clue is the 8 letter word INWARDLY
I
N
W
A
R
D
L
Y
More clues leading to the result INWARDLY
Buddy explains!

The straight part
We know that "Secretly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secretly = inwardly.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 INWARDLY mean?
INWARDLY adverb- In an inward manner; on the inside or to oneself.
- Completely, fully.