The most common and most likely answer to this crossword clue is the 8 letter word FENESTRA
I've seen this crossword clue in The Guardian
F
E
N
E
S
T
R
A
More clues leading to the result FENESTRA
- Marshland painting by Kent that’s all about light
- A Romeo comes back to home — female deprived of male first seen here?
- Spot iron weapon emplacement overlooking Argentina
- Opening refuge in wild that has no lake
- Fine English retreat, right with a window
- Fellow nearest broken window
- Accidentally refasten small opening
- Window crafted from iron set of tables by artist
- Refasten broken window
- Retreat in fear, breaking window
Buddy explains!

The straight part
We know that "small opening/small opening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case small opening/small opening = fenestra.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 FENESTRA mean?
FENESTRA noun- An opening in a body, sometimes with a membrane.