The most common and most likely answer to this crossword clue is the 6 letter word LATENT
I've seen this crossword clue in Independent
L
A
T
E
N
T
More clues leading to the result LATENT
- Overdue books waiting to be discovered
- Passed on religious books in secret
- Hidden in dead part of Australia
- Concealed former home of Olivier as it was
- Hidden talent, perhaps
- Potential to reveal fresh talent
- Not apparent in most recent books
- Previous set of books undiscovered
- Hidden talent for demolition
- Flat entrance partly concealed
Buddy explains!

The straight part
We know that "secret" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret = latent.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 LATENT mean?
LATENT noun- (forensics) The residue left by a person's finger that can be made visible by a process such as powder dusting; a latent fingerprint.
- An underlying cause that can be inferred from statistical correlations; factor.
- Anything that is latent.
- Existing or present but concealed or inactive.
- (of a virus) remaining in an inactive or hidden phase; dormant.
- Lying dormant or hidden until circumstances are suitable for development or manifestation.