The most common and most likely answer to this crossword clue is the 8 letter word MICRODOT
I've seen this crossword clue in The Daily Telegraph
M
I
C
R
O
D
O
T
More clues leading to the result MICRODOT
- One involved in Middle Common Room furore about LSD tablet
- Greatly reduced exposure of secret agent?
- Photograph of my bird Dorothy, say
- I’m returning to doctor awfully tiny photograph
- I’m repelled by staff in bed in secret photo
- Condensed material useful for a sleeper?
- Reduced exposure useful for moles?
- Military intelligence ordered doctor to reveal miniaturised picture
- Dangerous tablet with a small document
- Doctor doctor, I’m a photograph that needs processing to see
Buddy explains!

The straight part
We know that "secret photo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret photo = microdot.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 MICRODOT mean?
MICRODOT noun- A text or photographic image that has been reduced in size to that of a typographical dot in order to escape detection by unintended recipients.
- A small-sized tablet containing lysergic acid diethylamide (LSD).
- To make a microdot of.