The most common and most likely answer to this crossword clue is the 7 letter word MOONLIT
I've seen this crossword clue in Financial Times
M
O
O
N
L
I
T
More clues leading to the result MOONLIT
- Argue about Northern Line, one not seen in the brightest light
- Low dipstick gathering last drop of oil, visible when dark
- Cat returns to eat lion, maybe seen via satellite
- Low walls of national interest illuminated by night
- Show bottom student that item with heavenly illumination
- Low source of luminance one found in extremes of night — like this?
- Animated animal’s low note first shown by satellite
- Second one almost left the other glowing
- Doctor discussing literature describing romantic scenes?
- Erratic motion around Lagrange point as revealed by satellite
Buddy explains!

The straight part
We know that "illuminated at night" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illuminated at night = moonlit.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 MOONLIT mean?
MOONLIT adjective- Lit by moonlight.