The most common and most likely answer to this crossword clue is the 3 letter word PSI
I've seen this crossword clue in The Guardian
P
S
I
More clues leading to the result PSI
- Appearing in cap, sizeable Greek character
- Postie occasionally delivers foreign letter
- Greek character flops in shows
- Letter of alphabet representing measure of tyre inflation
- Letter from overseas company getting you online backing
- Greek character past it, oddly
- Letter from broadband provider turned up
- Athenian’s letter is upside-down in part
- Extra piece the author put in letter, section of second letter or third
- Letter and word game – endless twists
Buddy explains!
The straight part
We know that "third" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case third = psi.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 PSI mean?
PSI noun- The twenty-third letter of Classical and Modern Greek and the twenty-fifth letter of Old and Ancient Greek.
- A form of psychic energy.