The most common and most likely answer to this crossword clue is the 8 letter word INFRARED
I've seen this crossword clue in The Guardian
I
N
F
R
A
R
E
D
More clues leading to the result INFRARED
- Diner far off – it’s beyond a certain range?
- Radiation during and following unusual day
- Governing, managed to conceal source of riches — it’s invisible
- Find out about unusual end of spectrum
- Find out about unusual part of spectrum
- Find out about unusual type of radiation
- Such photography has popular female artist looking embarrassed
- Woolly ran if colour’s outside the visible spectrum
- Find out about exceptional light source
- Unusual find full of seldom-encountered radiation
Buddy explains!

The straight part
We know that "it’s invisible/it’s invisible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s invisible/it’s invisible = infrared.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 INFRARED mean?
INFRARED noun- Electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation, having a wavelength between 700 nm and 1 mm
- Having the wavelength in the infrared.
- In the infrared spectrum.